Add comprehensive tests for scheduled task repository, scheduler service, and task handlers

- Implemented tests for ScheduledTaskRepository covering task creation, retrieval, filtering, and status updates.
- Developed tests for SchedulerService including task creation, cancellation, user task retrieval, and maintenance jobs.
- Created tests for TaskHandlerRegistry to validate task execution for various types, including credit recharge and sound playback.
- Ensured proper error handling and edge cases in task execution scenarios.
- Added fixtures and mocks to facilitate isolated testing of services and repositories.

app/core/database.py

@@ -7,17 +7,8 @@ from sqlmodel.ext.asyncio.session import AsyncSession
 from app.core.config import settings
 from app.core.logging import get_logger
 from app.core.seeds import seed_all_data
-from app.models import (  # noqa: F401
-    extraction,
-    favorite,
-    plan,
-    playlist,
-    playlist_sound,
-    sound,
-    sound_played,
-    user,
-    user_oauth,
-)
+# Import all models to ensure SQLModel metadata discovery
+import app.models  # noqa: F401
 
 engine: AsyncEngine = create_async_engine(
     settings.DATABASE_URL,