- 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>
