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- 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>