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>

packages/db/src/schema.ts

@@ -463,6 +463,7 @@ export const conversations = pgTable(
     businessNumber: text("business_number").notNull(),
     lastMessageAt: timestamp("last_message_at"),
     status: text("status").notNull().default("active"),
+    staffReadAt: timestamp("staff_read_at"),
     createdAt: timestamp("created_at").notNull().defaultNow(),
     updatedAt: timestamp("updated_at").notNull().defaultNow(),
     staffReadAt: timestamp("staff_read_at"),