fix(assignments): delete_assignment used wrong column name on MonitoringSession

The safety check that refuses to delete assignments with real recording
history referenced MonitoringSession.start_time, but the actual column
is MonitoringSession.started_at.  Every DELETE call to /assignments/{id}
crashed with AttributeError before doing anything.

Now uses started_at correctly.  Verified end-to-end on dev.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/routers/project_locations.py

@@ -874,8 +874,8 @@ async def delete_assignment(
         and_(
             MonitoringSession.location_id == assignment.location_id,
             MonitoringSession.unit_id == assignment.unit_id,
-            MonitoringSession.start_time >= window_start,
-            MonitoringSession.start_time <= window_end,
+            MonitoringSession.started_at >= window_start,
+            MonitoringSession.started_at <= window_end,
         )
     ).count()