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)})