Fix broken joinedload and import path in reviews/activities endpoints

joinedload("reviewer") used a string instead of a class-bound
attribute (rejected by SQLAlchemy 2.x), and models.submission was the
wrong import path for Submission. Both endpoints 500'd whenever
actually called; surfaced while wiring real dashboard/activity data
into the frontend.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/utils/activity.py

@@ -302,10 +302,9 @@ class ActivityService:
         """Get activities excluding those related to deleted records."""
         from sqlalchemy import and_, or_, desc
         from datetime import datetime, timedelta
-        from models.task import Task
+        from models.task import Task, Submission
         from models.shot import Shot
         from models.asset import Asset
-        from models.submission import Submission
         
         query = db.query(Activity)