fix(auth): Fix DetachedInstanceError on dashboard for users with company associations

user_loader was closing the SQLAlchemy session after loading User,
causing lazy-load of company_associations to fail when dashboard template
calls can_edit_company(). Eagerly load associations in user_loader.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -320,10 +320,17 @@ logger.info("Blueprints registered")
 
 @login_manager.user_loader
 def load_user(user_id):
-    """Load user from database"""
+    """Load user from database with eager-loaded relationships"""
+    from sqlalchemy.orm import joinedload
     db = SessionLocal()
     try:
-        return db.query(User).filter_by(id=int(user_id)).first()
+        user = db.query(User).options(
+            joinedload(User.company_associations)
+        ).filter_by(id=int(user_id)).first()
+        if user:
+            # Force-load associations before detaching from session
+            _ = user.company_associations
+        return user
     finally:
         db.close()