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/user.py

@@ -58,3 +58,7 @@ class UserRepository:
         await self.db.commit()
         await self.db.refresh(user)
         return user
+
+    async def refresh(self, user: User) -> None:
+        """Refresh a user instance from the database."""
+        await self.db.refresh(user)