fix(GRO-1241): remove duplicate staffReadAt + add count mock

- Remove duplicate staffReadAt column in conversations table schema
  (merge conflict artifact — TS1117 duplicate definition)
- Add count mock to conversations.test.ts mock @groombook/db export
  (PR switched from sql\`count(*)\` to Drizzle count() without updating mock)

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

packages/db/src/schema.ts

@@ -466,7 +466,6 @@ export const conversations = pgTable(
     staffReadAt: timestamp("staff_read_at"),
     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(