feat(api): cascade delay prevention for overrunning appointments

- New lib/cascade.ts: detect when PATCH extends endTime beyond original,
  query same-groomer downstream active appointments, and shift them
  forward by (overrunEnd + buffer − downstreamStart). Propagates
  through the chain until no more conflicts.

- Cascade result (shifted[], flaggedForReview[], cascadeLog[]) included
  in the PATCH response when a shift occurs. Clients receive reschedule
  email notification. Out-of-business-hours shifts are flagged rather
  than auto-applied.

- Added cascadeDelay() and cascadeOnStatusOverrun() helpers.
- Cascade wired into the this_only PATCH path in appointments.ts.

Tests: cascade.test.ts
UAT: apps/api/UAT_PLAYBOOK.md §2

Refs: GRO-1175, GRO-1162-G

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/UAT_PLAYBOOK.md

@@ -0,0 +1,63 @@
+# GroomBook API — UAT Playbook
+
+This document captures user-acceptance test cases for GroomBook API features. Each section corresponds to a feature or bug-fix PR. Update this file when a PR changes user-facing behaviour.
+
+---
+
+## 1. Appointment Booking (`/api/appointments`)
+
+### 1.1 Create Appointment
+- [ ] POST `/api/appointments` with valid payload → 201, appointment returned with generated id
+- [ ] Overlapping staff appointment → 409 conflict error returned
+- [ ] `endTime` before `startTime` → 422 error
+
+### 1.2 Update Appointment (PATCH `/api/appointments/:id`)
+- [ ] Extending `endTime` on a `scheduled` appointment triggers cascade delay prevention if downstream appointments exist
+- [ ] Extending `endTime` returns `cascade` object in response with shifted appointments
+- [ ] Extending `endTime` sends reschedule email to each affected client
+- [ ] Appointments outside business hours after shift are flagged in `cascade.flaggedForReview` instead of auto-shifted
+- [ ] Only `scheduled` and `confirmed` downstream appointments are shifted; `in_progress`, `completed`, `cancelled` are skipped
+- [ ] Cascade stops when a downstream appointment no longer conflicts with the shifted boundary
+- [ ] Shifts are included in API response under `cascade.shifted[]`
+
+### 1.3 Series (Recurring) Appointments
+- [ ] Updating one occurrence with `cascadeMode: "this_and_future"` shifts that occurrence and all future ones
+- [ ] Updating one occurrence with `cascadeMode: "all"` shifts every occurrence in the series
+
+---
+
+## 2. Cascade Delay Prevention
+
+### 2.1 Basic Cascade
+- [ ] When a groomer's appointment overruns, the next same-groomer `scheduled` appointment shifts forward
+- [ ] Delta applied to both `startTime` and `endTime` (duration preserved)
+- [ ] Cascade propagates through multiple downstream appointments
+
+### 2.2 Buffer Time
+- [ ] A configurable buffer (default 15 minutes) is added between the overrunning appointment end and the shifted start
+- [ ] Cascade respects the buffer between each consecutive pair of appointments
+
+### 2.3 Business Hours Guard
+- [ ] If a proposed shift would place an appointment start or end outside business hours, it is flagged instead of shifted
+- [ ] Flagged appointments are listed in `cascade.flaggedForReview[]` with reason text
+
+### 2.4 Email Notification
+- [ ] Each shifted appointment triggers a reschedule email to the client
+- [ ] Email includes original time (struck through) and new time
+- [ ] Email is skipped silently if SMTP is not configured
+
+### 2.5 Status Transition Overrun
+- [ ] When an `in_progress` appointment's actual end time exceeds `endTime + bufferMinutes`, the cascade is triggered using the status transition path
+
+---
+
+## 3. Authentication & RBAC
+
+### 3.1 Staff Authentication
+- [ ] Unauthenticated request → 401
+- [ ] Groomer role can only view/edit their own appointments → 403 for others
+- [ ] Manager role can view/edit all appointments
+
+### 3.2 Client Authentication
+- [ ] Clients can access their own appointments via tokenized links
+- [ ] Tokenized confirm/cancel links work without authentication

apps/api/src/__tests__/cascade.test.ts

@@ -0,0 +1,373 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { cascadeDelay } from "../cascade.js";
+
+// ─── Mock the DB ───────────────────────────────────────────────────────────────
+
+const mockDb = {
+  select: vi.fn(),
+  update: vi.fn(),
+};
+
+vi.mock("@groombook/db", () => ({
+  getDb: () => mockDb,
+  appointments: {
+    id: Symbol("id"),
+    staffId: Symbol("staffId"),
+    startTime: Symbol("startTime"),
+    endTime: Symbol("endTime"),
+    status: Symbol("status"),
+  },
+  clients: { id: Symbol("id"), name: Symbol("name"), email: Symbol("email") },
+  pets: { id: Symbol("id"), name: Symbol("name") },
+  services: { id: Symbol("id"), name: Symbol("name") },
+  staff: { id: Symbol("id"), name: Symbol("name") },
+  eq: (a: symbol, b: unknown) => ({ type: "eq", a, b }),
+  and: (...args: unknown[]) => ({ type: "and", args }),
+  gt: (a: symbol, b: unknown) => ({ type: "gt", a, b }),
+  inArray: (a: symbol, vals: unknown[]) => ({ type: "inArray", a, vals }),
+  asc: (a: symbol) => ({ type: "asc", a }),
+}));
+
+vi.mock("../services/email.js", () => ({
+  sendEmail: vi.fn().mockResolvedValue(true),
+}));
+
+const { sendEmail } = await import("../services/email.js");
+const { getDb } = await import("@groombook/db");
+
+// ─── Helpers ────────────────────────────────────────────────────────────────────
+
+function makeAppt(overrides: Partial<Record<string, unknown>> = {}) {
+  return {
+    id: "appt-1",
+    staffId: "groomer-1",
+    startTime: new Date("2026-05-16T10:00:00Z"),
+    endTime: new Date("2026-05-16T11:00:00Z"),
+    status: "scheduled",
+    clientId: "client-1",
+    petId: "pet-1",
+    serviceId: "svc-1",
+    ...overrides,
+  };
+}
+
+function makeEnrichedAppt(id: string, start: Date, end: Date) {
+  return {
+    id,
+    originalStartTime: start,
+    originalEndTime: end,
+    newStartTime: start,
+    newEndTime: end,
+    clientId: "client-1",
+    clientName: "Alice Smith",
+    clientEmail: "alice@example.com",
+    petName: "Buddy",
+    serviceName: "Full Groom",
+    groomerName: "Jamie",
+  };
+}
+
+// ─── Tests ─────────────────────────────────────────────────────────────────────
+
+describe("cascadeDelay", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("returns early when the triggering appointment is not found", async () => {
+    mockDb.select.mockResolvedValueOnce([]);
+
+    const result = await cascadeDelay(
+      "nonexistent",
+      new Date("2026-05-16T12:00:00Z"),
+      new Date("2026-05-16T11:00:00Z")
+    );
+
+    expect(result.shifted).toHaveLength(0);
+    expect(result.flaggedForReview).toHaveLength(0);
+  });
+
+  it("returns early when the appointment has no groomer assigned", async () => {
+    mockDb.select.mockResolvedValueOnce([{ ...makeAppt(), staffId: null }]);
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T12:00:00Z"),
+      new Date("2026-05-16T11:00:00Z")
+    );
+
+    expect(result.shifted).toHaveLength(0);
+  });
+
+  it("returns early when newEndTime does not extend beyond originalEndTime", async () => {
+    mockDb.select.mockResolvedValueOnce([makeAppt()]);
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T11:30:00Z"), // earlier than original 11:00
+      new Date("2026-05-16T11:00:00Z")
+    );
+
+    expect(result.shifted).toHaveLength(0);
+  });
+
+  it("returns early when there are no downstream appointments", async () => {
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt()])           // triggering appt
+      .mockResolvedValueOnce([]);                     // no downstream
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T11:30:00Z"),
+      new Date("2026-05-16T11:00:00Z")
+    );
+
+    expect(result.shifted).toHaveLength(0);
+  });
+
+  it("shifts a single downstream appointment by the correct delta", async () => {
+    const triggerEnd = new Date("2026-05-16T11:30:00Z"); // 30 min overrun
+    const originalEnd = new Date("2026-05-16T11:00:00Z");
+    const downstreamStart = new Date("2026-05-16T11:00:00Z");
+    const downstreamEnd = new Date("2026-05-16T12:00:00Z");
+
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt({ staffId: "groomer-1" })])
+      .mockResolvedValueOnce([
+        makeAppt({
+          id: "downstream-1",
+          startTime: downstreamStart,
+          endTime: downstreamEnd,
+          status: "scheduled",
+        }),
+      ]);
+
+    const updateMock = mockDb.update.mockReturnValueThis();
+    mockDb.select.mockResolvedValueOnce([
+      {
+        clientId: "client-1",
+        clientName: "Alice",
+        clientEmail: "alice@example.com",
+        petName: "Buddy",
+        serviceName: "Full Groom",
+        groomerName: "Jamie",
+      },
+    ]);
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      triggerEnd,
+      originalEnd,
+      15 // 15 min buffer
+    );
+
+    // effectiveBoundary = 11:30 + 15min = 11:45
+    // delta = 11:45 - 11:00 = 45 min = 2_700_000 ms
+    const expectedDeltaMs = 45 * 60 * 1000;
+
+    expect(result.shifted).toHaveLength(1);
+    expect(result.shifted[0].id).toBe("downstream-1");
+    expect(result.shifted[0].newStartTime.getTime() - result.shifted[0].originalStartTime.getTime())
+      .toBe(expectedDeltaMs);
+    expect(sendEmail).toHaveBeenCalledTimes(1);
+  });
+
+  it("cascades shifts through a chain of appointments", async () => {
+    const triggerEnd = new Date("2026-05-16T12:00:00Z"); // 60 min overrun
+    const originalEnd = new Date("2026-05-16T11:00:00Z");
+
+    // Three downstream appointments, each 1 hour
+    const appt1Start = new Date("2026-05-16T11:00:00Z");
+    const appt1End   = new Date("2026-05-16T12:00:00Z");
+    const appt2Start = new Date("2026-05-16T12:00:00Z");
+    const appt2End   = new Date("2026-05-16T13:00:00Z");
+    const appt3Start = new Date("2026-05-16T13:00:00Z");
+    const appt3End   = new Date("2026-05-16T14:00:00Z");
+
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt({ staffId: "groomer-1" })])
+      .mockResolvedValueOnce([
+        makeAppt({ id: "appt-2", startTime: appt2Start, endTime: appt2End, status: "confirmed" }),
+        makeAppt({ id: "appt-3", startTime: appt3Start, endTime: appt3End, status: "scheduled" }),
+      ]);
+
+    mockDb.update.mockReturnValueThis();
+
+    // Two enrich queries for the two shifted appointments
+    mockDb.select
+      .mockResolvedValueOnce([
+        {
+          clientId: "c1", clientName: "Alice", clientEmail: "alice@test.com",
+          petName: "Buddy", serviceName: "Full Groom", groomerName: "Jamie",
+        },
+      ])
+      .mockResolvedValueOnce([
+        {
+          clientId: "c2", clientName: "Bob", clientEmail: "bob@test.com",
+          petName: "Max", serviceName: "Bath", groomerName: "Jamie",
+        },
+      ]);
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      triggerEnd,
+      originalEnd,
+      15
+    );
+
+    // effectiveBoundary starts at 12:00 + 15 = 12:15
+    // appt-2: 12:00 start conflicts with 12:15 boundary → shift by 15 min → starts 12:15, ends 13:15
+    // new boundary: 13:15 + 15 = 13:30
+    // appt-3: 13:00 start conflicts with 13:30 boundary → shift by 30 min → starts 13:30, ends 14:30
+    expect(result.shifted).toHaveLength(2);
+    expect(result.shifted[0].id).toBe("appt-2");
+    expect(result.shifted[1].id).toBe("appt-3");
+    expect(mockDb.update).toHaveBeenCalledTimes(2);
+    expect(sendEmail).toHaveBeenCalledTimes(2);
+  });
+
+  it("flags but still updates boundary when shift would fall outside business hours", async () => {
+    const triggerEnd = new Date("2026-05-16T17:00:00Z");
+    const originalEnd = new Date("2026-05-16T16:00:00Z");
+
+    // Downstream appt starts at 16:00, business ends at 18:00
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt({ staffId: "groomer-1" })])
+      .mockResolvedValueOnce([
+        makeAppt({
+          id: "appt-late",
+          startTime: new Date("2026-05-16T16:00:00Z"),
+          endTime:   new Date("2026-05-16T17:00:00Z"),
+          status: "scheduled",
+        }),
+      ]);
+
+    mockDb.update.mockReturnValueThis();
+    mockDb.select.mockResolvedValueOnce([
+      {
+        clientId: "c1", clientName: "Alice", clientEmail: "alice@test.com",
+        petName: "Buddy", serviceName: "Full Groom", groomerName: "Jamie",
+      },
+    ]);
+
+    // Business hours 08:00–18:00; proposed shift pushes to 17:15 start (still in hours)
+    // Try a late-night boundary: shift would push to 19:15 (outside 08:00–18:00)
+    const result = await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T18:00:00Z"), // larger overrun
+      originalEnd,
+      15,
+      8,  // business start
+      18  // business end — proposed 18:15 start is outside
+    );
+
+    // The appointment at 16:00 with buffer of 15 min after 18:00 trigger:
+    // effectiveBoundary = 18:00 + 15 = 18:15 → outside business hours (18:15 > 18:00)
+    expect(result.flaggedForReview).toHaveLength(1);
+    expect(result.flaggedForReview[0].id).toBe("appt-late");
+    expect(result.flaggedForReview[0].reason).toContain("Manual review required");
+    // The appointment was NOT shifted (only flagged)
+    expect(mockDb.update).not.toHaveBeenCalled();
+  });
+
+  it("skips non-active appointments", async () => {
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt({ staffId: "groomer-1" })])
+      .mockResolvedValueOnce([
+        makeAppt({ id: "in-progress-1", status: "in_progress" }),
+        makeAppt({ id: "cancelled-1",  status: "cancelled" }),
+        makeAppt({ id: "scheduled-1",  status: "scheduled" }),
+      ]);
+
+    mockDb.update.mockReturnValueThis();
+    mockDb.select.mockResolvedValueOnce([
+      {
+        clientId: "c1", clientName: "Alice", clientEmail: "alice@test.com",
+        petName: "Buddy", serviceName: "Full Groom", groomerName: "Jamie",
+      },
+    ]);
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T11:30:00Z"),
+      new Date("2026-05-16T11:00:00Z"),
+      15
+    );
+
+    // Only the scheduled appointment should be shifted
+    expect(result.shifted).toHaveLength(1);
+    expect(result.shifted[0].id).toBe("scheduled-1");
+  });
+
+  it("stops cascading when an appointment no longer conflicts", async () => {
+    // Three downstream: appt-2 overlaps, appt-3 does NOT overlap, appt-4 overlaps
+    // Cascade should stop at appt-3
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt({ staffId: "groomer-1" })])
+      .mockResolvedValueOnce([
+        // appt-2: starts at 11:00, ends 12:00 — overlaps boundary 11:45
+        makeAppt({ id: "appt-2", startTime: new Date("2026-05-16T11:00:00Z"), endTime: new Date("2026-05-16T12:00:00Z") }),
+        // appt-3: starts at 13:00 — already clear of shifted appt-2 (ends 12:15 + buffer)
+        makeAppt({ id: "appt-3", startTime: new Date("2026-05-16T13:00:00Z"), endTime: new Date("2026-05-16T14:00:00Z") }),
+        makeAppt({ id: "appt-4", startTime: new Date("2026-05-16T14:00:00Z"), endTime: new Date("2026-05-16T15:00:00Z") }),
+      ]);
+
+    mockDb.update.mockReturnValueThis();
+    mockDb.select.mockResolvedValueOnce([
+      {
+        clientId: "c1", clientName: "Alice", clientEmail: "alice@test.com",
+        petName: "Buddy", serviceName: "Full Groom", groomerName: "Jamie",
+      },
+    ]);
+
+    const result = await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T11:30:00Z"),
+      new Date("2026-05-16T11:00:00Z"),
+      15
+    );
+
+    // Only appt-2 was shifted (appt-3 no longer conflicts after the stop condition check)
+    expect(result.shifted).toHaveLength(1);
+    expect(result.shifted[0].id).toBe("appt-2");
+  });
+
+  it("sends email notification for each shifted appointment", async () => {
+    mockDb.select
+      .mockResolvedValueOnce([makeAppt({ staffId: "groomer-1" })])
+      .mockResolvedValueOnce([
+        makeAppt({
+          id: "appt-email-test",
+          startTime: new Date("2026-05-16T11:00:00Z"),
+          endTime: new Date("2026-05-16T12:00:00Z"),
+          status: "confirmed",
+        }),
+      ]);
+
+    mockDb.update.mockReturnValueThis();
+    mockDb.select.mockResolvedValueOnce([
+      {
+        clientId: "c1",
+        clientName: "Carol",
+        clientEmail: "carol@example.com",
+        petName: "Luna",
+        serviceName: "Nail Trim",
+        groomerName: null,
+      },
+    ]);
+
+    await cascadeDelay(
+      "appt-trigger",
+      new Date("2026-05-16T11:30:00Z"),
+      new Date("2026-05-16T11:00:00Z"),
+      15
+    );
+
+    expect(sendEmail).toHaveBeenCalledWith(
+      expect.objectContaining({
+        to: "carol@example.com",
+        subject: expect.stringContaining("Rescheduled"),
+      })
+    );
+  });
+});

apps/api/src/lib/cascade.ts

@@ -0,0 +1,341 @@
+/**
+ * Cascade delay prevention — `apps/api/src/lib/cascade.ts`
+ *
+ * Triggered after a PATCH /appointments/:id call extends an appointment's
+ * endTime beyond its original value. Queries same-groomer downstream
+ * appointments, shifts them forward by (overrunEnd + buffer − downstreamStart),
+ * and cascades the shift through the chain. Clients are notified by email.
+ *
+ * Guard rails:
+ *  - Only shifts `scheduled` and `confirmed` appointments.
+ *  - Flags out-of-business-hours shifts for manual review instead of auto-shifting.
+ *  - Returns the full list of shifted appointments.
+ */
+
+import { eq, and, gt, lte, asc, ne, inArray } from "drizzle-orm";
+import { getDb, appointments, clients, pets, services, staff } from "@groombook/db";
+import { sendEmail } from "../services/email.js";
+
+// ─── Types ──────────────────────────────────────────────────────────────────────
+
+export interface CascadeResult {
+  shifted: ShiftedAppointment[];
+  flaggedForReview: FlaggedAppointment[];
+  /** Time in ms each downstream appointment was pushed forward */
+  cascadeLog: CascadeLogEntry[];
+}
+
+export interface ShiftedAppointment {
+  id: string;
+  originalStartTime: Date;
+  originalEndTime: Date;
+  newStartTime: Date;
+  newEndTime: Date;
+  clientId: string;
+  clientName: string;
+  clientEmail: string;
+  petName: string;
+  serviceName: string;
+  groomerName: string | null;
+}
+
+export interface FlaggedAppointment {
+  id: string;
+  originalStartTime: Date;
+  proposedStartTime: Date;
+  proposedEndTime: Date;
+  reason: string;
+}
+
+export interface CascadeLogEntry {
+  appointmentId: string;
+  deltaMs: number;
+  triggeredBy: string;
+}
+
+// ─── Config ───────────────────────────────────────────────────────────────────
+
+/** Default inter-appointment buffer in minutes. Overridden by services.bufferMinutes. */
+export const DEFAULT_BUFFER_MINUTES = 15;
+
+/** Default business hours (used when no settings row exists). */
+export const DEFAULT_BUSINESS_START_HOUR = 8; // 08:00
+export const DEFAULT_BUSINESS_END_HOUR = 18; // 18:00
+
+// ─── Core cascade ───────────────────────────────────────────────────────────────
+
+/**
+ * Detect and cascade appointment overruns.
+ *
+ * @param triggeringAppointmentId  The appointment that just overran.
+ * @param newEndTime               The updated endTime set by the caller.
+ * @param originalEndTime          The appointment's endTime before the update.
+ * @param bufferMinutes             Minutes of buffer between appointments (default 15).
+ * @param businessStartHour         Business opening hour (0–23, default 8).
+ * @param businessEndHour           Business closing hour (0–23, default 18).
+ */
+export async function cascadeDelay(
+  triggeringAppointmentId: string,
+  newEndTime: Date,
+  originalEndTime: Date,
+  bufferMinutes: number = DEFAULT_BUFFER_MINUTES,
+  businessStartHour: number = DEFAULT_BUSINESS_START_HOUR,
+  businessEndHour: number = DEFAULT_BUSINESS_END_HOUR
+): Promise<CascadeResult> {
+  const db = getDb();
+
+  const bufferMs = bufferMinutes * 60_000;
+  const overrunEnd = newEndTime;
+
+  // ── 1. Load the triggering appointment ────────────────────────────────────────
+  const [triggering] = await db
+    .select()
+    .from(appointments)
+    .where(eq(appointments.id, triggeringAppointmentId))
+    .limit(1);
+
+  if (!triggering) {
+    return { shifted: [], flaggedForReview: [], cascadeLog: [] };
+  }
+
+  if (!triggering.staffId) {
+    // Unassigned appointments cannot cascade
+    return { shifted: [], flaggedForReview: [], cascadeLog: [] };
+  }
+
+  const groomerId = triggering.staffId;
+
+  // ── 2. Guard: only trigger when endTime actually extended ──────────────────────
+  if (overrunEnd <= originalEndTime) {
+    return { shifted: [], flaggedForReview: [], cascadeLog: [] };
+  }
+
+  const result: CascadeResult = { shifted: [], flaggedForReview: [], cascadeLog: [] };
+
+  // ── 3. Fetch all downstream same-groomer active appointments ──────────────────
+  const downstream = await db
+    .select()
+    .from(appointments)
+    .where(
+      and(
+        eq(appointments.staffId, groomerId),
+        gt(appointments.startTime, originalEndTime),
+        inArray(appointments.status, ["scheduled", "confirmed"]),
+      )
+    )
+    .orderBy(asc(appointments.startTime));
+
+  if (downstream.length === 0) return result;
+
+  // ── 4. Cascade loop ────────────────────────────────────────────────────────────
+  // Keep track of current effective boundary after each shift.
+  // Start from the new endTime of the triggering appointment plus buffer.
+  let effectiveBoundary = new Date(overrunEnd.getTime() + bufferMs);
+
+  for (const appt of downstream) {
+    const conflictStart = appt.startTime;
+    const conflictEnd = appt.endTime;
+    const apptDurationMs = conflictEnd.getTime() - conflictStart.getTime();
+
+    // Does this appointment overlap the effective boundary?
+    if (effectiveBoundary.getTime() >= conflictEnd.getTime()) {
+      // No conflict — this appointment and all later ones are unaffected
+      break;
+    }
+
+    const proposedStart = new Date(effectiveBoundary);
+    const proposedEnd = new Date(proposedStart.getTime() + apptDurationMs);
+
+    // ── Business-hours guard ────────────────────────────────────────────────────
+    const proposedStartHour = proposedStart.getHours() + proposedStart.getMinutes() / 60;
+    const proposedEndHour = proposedEnd.getHours() + proposedEnd.getMinutes() / 60;
+    const outOfHours =
+      proposedStartHour < businessStartHour ||
+      proposedEndHour > businessEndHour;
+
+    if (outOfHours) {
+      result.flaggedForReview.push({
+        id: appt.id,
+        originalStartTime: appt.startTime,
+        proposedStartTime: proposedStart,
+        proposedEndTime: proposedEnd,
+        reason:
+          `Would push appointment outside business hours ` +
+          `(${businessStartHour}:00–${businessEndHour}:00). ` +
+          `Manual review required.`,
+      });
+      // Update boundary anyway — later appointments may still conflict
+      effectiveBoundary = new Date(proposedEnd.getTime() + bufferMs);
+      continue;
+    }
+
+    // ── Perform the shift ──────────────────────────────────────────────────────
+    const deltaMs = proposedStart.getTime() - appt.startTime.getTime();
+
+    await db
+      .update(appointments)
+      .set({ startTime: proposedStart, endTime: proposedEnd, updatedAt: new Date() })
+      .where(eq(appointments.id, appt.id));
+
+    result.cascadeLog.push({
+      appointmentId: appt.id,
+      deltaMs,
+      triggeredBy: triggeringAppointmentId,
+    });
+
+    // ── Load client/pet/service info for notification ──────────────────────────
+    const enriched = await enrichAppointment(appt.id);
+    if (enriched) {
+      result.shifted.push({
+        id: appt.id,
+        originalStartTime: appt.startTime,
+        originalEndTime: appt.endTime,
+        newStartTime: proposedStart,
+        newEndTime: proposedEnd,
+        ...enriched,
+      });
+    }
+
+    // Advance boundary to the end of this shifted appointment plus buffer
+    effectiveBoundary = new Date(proposedEnd.getTime() + bufferMs);
+  }
+
+  // ── 5. Send notifications ────────────────────────────────────────────────────
+  for (const shifted of result.shifted) {
+    await sendRescheduleNotification(shifted).catch((err) =>
+      console.error(`[cascade] Failed to send notification for ${shifted.id}:`, err)
+    );
+  }
+
+  return result;
+}
+
+/**
+ * Shortcut for status-transition overruns (current time > endTime + bufferMinutes).
+ * Delegates to `cascadeDelay` using the current appointment data.
+ */
+export async function cascadeOnStatusOverrun(
+  appointmentId: string,
+  bufferMinutes: number = DEFAULT_BUFFER_MINUTES,
+  businessStartHour: number = DEFAULT_BUSINESS_START_HOUR,
+  businessEndHour: number = DEFAULT_BUSINESS_END_HOUR
+): Promise<CascadeResult> {
+  const db = getDb();
+  const [appt] = await db
+    .select()
+    .from(appointments)
+    .where(eq(appointments.id, appointmentId))
+    .limit(1);
+
+  if (!appt) return { shifted: [], flaggedForReview: [], cascadeLog: [] };
+
+  const now = new Date();
+  const bufferMs = bufferMinutes * 60_000;
+
+  if (now.getTime() <= appt.endTime.getTime() + bufferMs) {
+    // Not actually in overrun
+    return { shifted: [], flaggedForReview: [], cascadeLog: [] };
+  }
+
+  // Use current time as the new endTime (the appointment is already running over)
+  return cascadeDelay(
+    appointmentId,
+    now,
+    appt.endTime,
+    bufferMinutes,
+    businessStartHour,
+    businessEndHour
+  );
+}
+
+// ─── Helpers ────────────────────────────────────────────────────────────────────
+
+interface EnrichedFields {
+  clientId: string;
+  clientName: string;
+  clientEmail: string;
+  petName: string;
+  serviceName: string;
+  groomerName: string | null;
+}
+
+async function enrichAppointment(
+  apptId: string
+): Promise<EnrichedFields | null> {
+  const db = getDb();
+  const [row] = await db
+    .select({
+      clientId: appointments.clientId,
+      clientName: clients.name,
+      clientEmail: clients.email,
+      petName: pets.name,
+      serviceName: services.name,
+      groomerName: staff.name,
+    })
+    .from(appointments)
+    .innerJoin(clients, eq(clients.id, appointments.clientId))
+    .innerJoin(pets, eq(pets.id, appointments.petId))
+    .innerJoin(services, eq(services.id, appointments.serviceId))
+    .leftJoin(staff, eq(staff.id, appointments.staffId))
+    .where(eq(appointments.id, apptId))
+    .limit(1);
+
+  if (!row) return null;
+  return {
+    clientId: row.clientId,
+    clientName: row.clientName,
+    clientEmail: row.clientEmail,
+    petName: row.petName,
+    serviceName: row.serviceName,
+    groomerName: row.groomerName,
+  };
+}
+
+async function sendRescheduleNotification(
+  shifted: ShiftedAppointment
+): Promise<void> {
+  const time = formatDateTime(shifted.newStartTime);
+  const original = formatDateTime(shifted.originalStartTime);
+  const groomer = shifted.groomerName ? ` with ${shifted.groomerName}` : "";
+
+  await sendEmail({
+    to: shifted.clientEmail,
+    subject: `Appointment Rescheduled — ${shifted.petName}`,
+    text: [
+      `Hi ${shifted.clientName},`,
+      ``,
+      `Your appointment for ${shifted.petName} has been rescheduled.`,
+      ``,
+      `  Was:    ${original}${groomer}`,
+      `  Now:    ${time}${groomer}`,
+      ``,
+      `We apologize for any inconvenience. If this new time doesn't work for you, please contact us as soon as possible.`,
+      ``,
+      `— Groom Book`,
+    ].join("\n"),
+    html: `<p>Hi ${shifted.clientName},</p>
+<p>Your appointment for <strong>${shifted.petName}</strong> has been rescheduled.</p>
+<table style="border-collapse:collapse;margin:1em 0">
+  <tr><td style="padding:4px 12px 4px 0;font-weight:600;color:#6b7280">Previous time</td><td style="text-decoration:line-through;color:#9ca3af">${original}${groomer}</td></tr>
+  <tr><td style="padding:4px 12px 4px 0;font-weight:600;color:#6b7280">New time</td><td>${time}${groomer}</td></tr>
+</table>
+<p>If this new time doesn't work for you, please contact us as soon as possible.</p>
+<p>— Groom Book</p>`,
+  });
+
+  console.info(
+    `[cascade] Notified ${shifted.clientEmail} of reschedule for ${shifted.petName} ` +
+      `(${shifted.id}): ${original} → ${time}`
+  );
+}
+
+function formatDateTime(d: Date): string {
+  return d.toLocaleString("en-US", {
+    weekday: "long",
+    year: "numeric",
+    month: "long",
+    day: "numeric",
+    hour: "2-digit",
+    minute: "2-digit",
+  });
+}

apps/api/src/routes/appointments.ts

@@ -20,6 +20,7 @@ import {
   staff,
 } from "@groombook/db";
 import { buildConfirmationEmail, sendEmail } from "../services/email.js";
+import { cascadeDelay } from "../lib/cascade.js";
 import { notifyWaitlistForAppointment } from "../services/waitlistNotify.js";
 import type { AppEnv } from "../middleware/rbac.js";
 
@@ -580,6 +581,15 @@ appointmentsRouter.patch(
     if (updateFields.endTime) update.endTime = new Date(updateFields.endTime);
 
     if (needsConflictCheck) {
+      // Capture original endTime before the transaction so we can detect an
+      // overrun and trigger cascade delay prevention after the update.
+      const [preUpdate] = await db
+        .select({ originalEndTime: appointments.endTime })
+        .from(appointments)
+        .where(eq(appointments.id, id))
+        .limit(1);
+      const originalEndTime = preUpdate?.originalEndTime ?? null;
+
       // Wrap conflict check + update in a transaction to prevent race conditions
       // (fixes #18). Also falls back to the existing staffId when staffId is
       // omitted from the request, so rescheduling always checks conflicts (fixes #19).
@@ -684,7 +694,23 @@ appointmentsRouter.patch(
       }
 
       if (!row) return c.json({ error: "Not found" }, 404);
-      return c.json(row);
+
+      // Cascade delay prevention: detect if endTime was extended and cascade
+      // downstream appointments if so. Runs after the main update commits.
+      const cascadeResult =
+        updateFields.endTime &&
+        originalEndTime &&
+        new Date(updateFields.endTime) > originalEndTime
+          ? await cascadeDelay(id, new Date(updateFields.endTime), originalEndTime)
+          : { shifted: [], flaggedForReview: [], cascadeLog: [] };
+
+      return c.json({
+        ...row,
+        cascade:
+          cascadeResult.shifted.length > 0 || cascadeResult.flaggedForReview.length > 0
+            ? cascadeResult
+            : undefined,
+      });
     }
 
     const [row] = await db