feat(GRO-106): staff messages page

- Adds staff conversations API (GET /api/conversations, GET /api/conversations/:id/messages, POST /api/conversations/:id/messages) with auth scoping and cross-tenant protection
- Adds staffReadAt column to conversations table for unread tracking
- Adds staff Messages page with two-column inbox layout (thread list + conversation view + composer)
- Adds Messages entry to staff sidebar navigation
- Includes tests for the MessagesPage component

Part of GRO-106 (SMS/MMS integration) Phase 1.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/api/src/index.ts

@@ -8,6 +8,7 @@ import { petsRouter } from "./routes/pets.js";
 import { servicesRouter } from "./routes/services.js";
 import { appointmentsRouter } from "./routes/appointments.js";
 import { waitlistRouter } from "./routes/waitlist.js";
+import { conversationsRouter } from "./routes/conversations.js";
 import { portalRouter } from "./routes/portal.js";
 import { staffRouter } from "./routes/staff.js";
 import { invoicesRouter } from "./routes/invoices.js";
@@ -264,6 +265,7 @@ api.route("/pets", petsRouter);
 api.route("/services", servicesRouter);
 api.route("/appointments", appointmentsRouter);
 api.route("/waitlist", waitlistRouter);
+api.route("/conversations", conversationsRouter);
 api.route("/staff", staffRouter);
 api.route("/invoices", invoicesRouter);
 api.route("/reports", reportsRouter);