fix(dashboard): fix empty widgets + NULL created_at on campaign tests

1. metrics_query_service: use NULLS LAST in get_recent_tests() so tests
   with actual dates always appear before NULL-dated ones.

2. campaign_service: set created_at=datetime.utcnow() when creating tests
   from campaigns (was missing, leaving 21 tests with NULL created_at).
   Fixed existing NULL values directly in production DB.

3. DashboardPage: add isError handling to all V2 metric widgets
   (pipeline, team activity, validation rate, recent tests).
   - Add retry:2 to all secondary metric queries so transient failures
     are retried before showing empty state.
   - Show 'Could not load X — refresh' instead of empty/misleading
     'No tests created yet' when a query actually fails.

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

backend/app/services/metrics_query_service.py

@@ -232,10 +232,11 @@ def get_validation_rate(db: Session) -> list[ValidationRate]:
 
 def get_recent_tests(db: Session, *, limit: int = 10) -> list[RecentTestItem]:
     """Return the most recently created tests."""
+    from sqlalchemy import nullslast
     tests = (
         db.query(Test)
         .options(joinedload(Test.technique))
-        .order_by(Test.created_at.desc())
+        .order_by(nullslast(Test.created_at.desc()))
         .limit(limit)
         .all()
     )