Delegate user persistence to UserRepository

- Add create() and update() methods to UserRepository
- Update ProfileService to use repository.update()
- Update AuthService to use repository.create()

backend/services/auth.py

@@ -72,8 +72,8 @@ class AuthService:
         if default_role:
             user.roles.append(default_role)
 
-        self.db.add(user)
-        await self.db.flush()  # Get user ID
+        # Create user (flush to get ID)
+        user = await self.user_repo.create(user)
 
         # Mark invite as spent
         invite.status = InviteStatus.SPENT