Add support for fetching and synchronizing empty classroom data. This includes new protobuf definitions for task payloads and results, database models, repository, service, and HTTP handlers.
- Add `TASK_TYPE_GET_EMPTY_CLASSROOM` to `TaskType` enum
- Implement `EmptyClassroom` model and auto-migration
- Add `EmptyClassroomHandler` with `GET` and `POST /sync` routes
- Implement `EmptyClassroomSyncService` and `EmptyClassroomRepository`
- Update dependency injection with wire sets
