fix: SQLAlchemy async lazy loading for new users

Initialize user.roles=[] and user.preferences on new user creation
to avoid MissingGreenlet error in async context.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/domains/auth/service.py

@@ -130,11 +130,14 @@ class AuthService:
                 avatar_url=token_info.picture,
                 last_login=datetime.now(timezone.utc),
             )
+            # Initialize relationships to avoid lazy loading issues in async
+            user.roles = []
             self.session.add(user)
             await self.session.flush()  # Get the user ID
 
-            # Create default preferences
+            # Create default preferences and attach to user
             preferences = UserPreferences(user_id=user.id)
+            user.preferences = preferences
             self.session.add(preferences)
 
             logger.info(f"Created new user: {token_info.email}")