fix: handle role as str or Enum in audit_logger

user.role stored as plain string in MongoDB — calling .value on it
caused AttributeError on every login, blocking all auth.

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

backend/app/services/audit_logger.py

@@ -79,7 +79,7 @@ class AuditLogger:
             description=description,
             user_id=user.id if user else None,
             user_email=user.email if user else None,
-            user_role=user.role.value if user else None,
+            user_role=(user.role.value if hasattr(user.role, "value") else user.role) if user else None,
             ip_address=ip_address,
             user_agent=user_agent,
             request_id=request_id,