Refactor backend to adopt async patterns across services, API routes, and CRUD operations

- Migrated database sessions and operations to `AsyncSession` for full async support.
- Updated all service methods and dependencies (`get_db` to `get_async_db`) to support async logic.
- Refactored admin, user, organization, session-related CRUD methods, and routes with await syntax.
- Improved consistency and performance with async SQLAlchemy patterns.
- Enhanced logging and error handling for async context.

backend/app/core/database_async.py

@@ -159,6 +159,10 @@ async def check_async_database_health() -> bool:
         return False
 
 
+# Alias for consistency with main.py
+check_database_health = check_async_database_health
+
+
 async def init_async_db() -> None:
     """
     Initialize async database tables.