Complete repository delegation - remove remaining direct db operations

- Add commit() method to AvailabilityRepository for transaction control
- Add refresh() method to UserRepository
- Update AvailabilityService to use repository.commit() instead of db.commit()
- Update AuthService to use UserRepository.refresh() instead of db.refresh()
- All services now consistently delegate ALL persistence to repositories

backend/repositories/availability.py

@@ -64,3 +64,7 @@ class AvailabilityRepository:
         for availability in availabilities:
             self.db.add(availability)
         await self.db.flush()
+
+    async def commit(self) -> None:
+        """Commit the current transaction."""
+        await self.db.commit()