fix: correct ActivityLog import name in user deletion

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

app/api/routers/users.py

@@ -222,7 +222,7 @@ def _reassign_user_references(db: Session, old_id: int, new_id: int) -> None:
     project memberships)."""
     from app.models.apikey import APIKey
     from app.models.notification import Notification
-    from app.models.activity import Activity
+    from app.models.activity import ActivityLog as Activity
     from app.models.worklog import WorkLog as WorkLogModel
     from app.models.meeting import Meeting, MeetingParticipant
     from app.models.task import Task