feat(GRO-1208): add staff conversations API route and staffReadAt migration

- Add `staffReadAt` column to conversations table schema
- Add migration 0032_staff_read_at.sql for the new column
- Create /api/conversations router with GET / (list), GET /:id/messages (paginated), POST /:id/messages (send)
- Mark conversations as read (staffReadAt = NOW()) when staff fetches messages
- Return 409 when client has opted out of SMS
- 404 on cross-tenant access
- Add conversations.test.ts covering all 5 acceptance criteria

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

packages/db/migrations/0032_staff_read_at.sql

@@ -0,0 +1 @@
+ALTER TABLE "conversations" ADD COLUMN "staff_read_at" timestamp;