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/auth.py

@@ -79,9 +79,10 @@ class AuthService:
         invite.status = InviteStatus.SPENT
         invite.used_by_id = user.id
         invite.spent_at = datetime.now(UTC)
+        await self.invite_repo.update(invite)
 
-        await self.db.commit()
-        await self.db.refresh(user)
+        # Refresh user to ensure it's up to date
+        await self.user_repo.refresh(user)
 
         # Create access token
         access_token = create_access_token(data={"sub": str(user.id)})

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()