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/services/availability.py

@@ -118,7 +118,7 @@ class AvailabilityService:
             for slot in slots
         ]
         await self.availability_repo.create_multiple(availabilities)
-        await self.db.commit()
+        await self.availability_repo.commit()
 
         return AvailabilityDay(date=target_date, slots=slots)
 
@@ -188,7 +188,7 @@ class AvailabilityService:
                 )
 
             # Commit all changes atomically
-            await self.db.commit()
+            await self.availability_repo.commit()
         except Exception:
             # Rollback on any error to maintain atomicity
             await self.db.rollback()