Fix user audit trail logging stale agency value for old and new

Expire the SQLAlchemy cached user object after flush() so the
subsequent get_by_id() reloads the agency relationship with fresh
data. Previously the identity map returned the same Python object
with the old .agency, causing audit logs to record identical old
and new values on agency changes.

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

backend/app/repositories/user_repository.py

@@ -116,7 +116,7 @@ class UserRepository:
         if agency_id is not ...:
             user.agency_id = agency_id
         await self.session.flush()
-        # Re-fetch to get agency relationship loaded
+        self.session.expire(user)  # Clear cached state so relationship reloads
         return await self.get_by_id(user_id)
 
     async def list_agencies(self) -> list[Agency]: