Unify database schema: Resolve all User model conflicts and auth table incompatibilities

Major changes:
- Consolidate 3 different User models into single unified model (models/user.py)
- Use UUID primary keys throughout (matches existing database schema)
- Add comprehensive authentication fields while preserving existing data
- Remove duplicate User model from auth.py, keep APIKey/RefreshToken/TokenBlacklist
- Update all imports to use unified User model consistently
- Create database migration (002_add_auth_fields.sql) for safe schema upgrade
- Fix frontend User interface to handle UUID string IDs
- Add backward compatibility fields (name property, role field)
- Maintain relationships for authentication features (api_keys, refresh_tokens)

Schema conflicts resolved:
✅ Migration schema (UUID, 7 fields) + Basic model (Integer, 6 fields) + Auth model (Integer, 10 fields)
   → Unified model (UUID, 12 fields with full backward compatibility)
✅ Field inconsistencies (name vs full_name) resolved with compatibility property
✅ Database foreign key constraints updated for UUID relationships
✅ JWT token handling fixed for UUID user IDs

This completes the holistic database schema unification requested after quick
patching caused conflicts. All existing data preserved, full auth system functional.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

backend/app/core/auth_deps.py

@@ -61,7 +61,7 @@ def get_current_active_user(
     db: Session = Depends(get_db),
 ) -> Dict[str, Any]:
     """Get current authenticated and active user."""
-    from app.models.auth import User
+    from app.models.user import User
     
     user = db.query(User).filter(User.id == user_context["user_id"]).first()
     if not user: