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>

apps/api/src/__tests__/conversations.test.ts

@@ -169,6 +169,7 @@ vi.mock("@groombook/db", () => {
     lt: vi.fn((a, b) => ({ type: "lt", a, b })),
     sql: vi.fn(() => ({ __type: "sql" })),
     isNull: vi.fn((col) => ({ type: "isNull", col })),
+    count: vi.fn((col) => ({ type: "count", col })),
   };
 });