fix(GRO-637): invoice status transitions, tip-split validation, refund idempotency, and tip-split response format

* Fix invoice status transitions, tip-split validation, refund idempotency, and tip-split response format

- Add ALLOWED_TRANSITIONS state machine for invoice status changes (GRO-637)
- Replace floating-point tip-split validation with integer basis-points math
- Add idempotency key support to refund endpoint with new refunds table
- Return full invoice shape from POST /:id/tip-splits matching GET response
- All existing tests pass

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

* fix(invoices): wrap refund flow in transaction for idempotency safety

- Wrap idempotency check + processRefund() + db.insert() in db.transaction()
- This prevents duplicate Stripe refunds if the DB insert fails after Stripe processes the refund
- Add migration 0027_refunds for the refunds table (was missing)
- Removes out-of-scope changes from PR #278 (csrf.ts, appointmentGroups, appointments, book, groomingLogs, services, stripe-webhooks)

Fixes GRO-637 per CTO review

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

* fix(api): wire up CSRF middleware for protected routes

Register csrfMiddleware in the protected API routes after authMiddleware
and resolveStaffMiddleware to protect against CSRF attacks on state-
changing operations (POST, PUT, PATCH, DELETE).

Addresses CTO review feedback on PR #278.

* fix(api): remove CSRF middleware that breaks POST/PUT/PATCH/DELETE

The CSRF middleware requires x-csrf-token header but the frontend never
sends it, which would break all mutating operations with 403 errors.

CSRF protection should be implemented in a separate coordinated PR with
frontend changes.

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

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>
Co-authored-by: Flea Flicker <flea-flicker@groombook.ai>

packages/db/src/schema.ts

@@ -300,6 +300,25 @@ export const invoiceTipSplits = pgTable(
   (t) => [index("idx_invoice_tip_splits_invoice_id").on(t.invoiceId)]
 );
 
+// Refund records with idempotency key support
+export const refunds = pgTable(
+  "refunds",
+  {
+    id: uuid("id").primaryKey().defaultRandom(),
+    invoiceId: uuid("invoice_id")
+      .notNull()
+      .references(() => invoices.id, { onDelete: "restrict" }),
+    stripeRefundId: text("stripe_refund_id").notNull(),
+    idempotencyKey: text("idempotency_key").unique(),
+    amountCents: integer("amount_cents"),
+    createdAt: timestamp("created_at").notNull().defaultNow(),
+  },
+  (t) => [
+    index("idx_refunds_invoice_id").on(t.invoiceId),
+    index("idx_refunds_idempotency_key").on(t.idempotencyKey),
+  ]
+);
+
 // Tracks which reminder emails have been sent per appointment (prevents duplicates).
 // reminder_type values: "confirmation", "24h", "2h"
 export const reminderLogs = pgTable(