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/src/schema.ts

@@ -444,6 +444,7 @@ export const conversations = pgTable(
     status: text("status").notNull().default("active"),
     createdAt: timestamp("created_at").notNull().defaultNow(),
     updatedAt: timestamp("updated_at").notNull().defaultNow(),
+    staffReadAt: timestamp("staff_read_at"),
   },
   (t) => [
     index("idx_conversations_business_id_last_message_at").on(