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/campaign_service.py

@@ -181,6 +181,7 @@ def generate_campaign_from_threat_actor(
             tool_used=template.tool_suggested,
             created_by=user.id,
             state=TestState.draft,
+            created_at=datetime.utcnow(),
         )
         db.add(test)
         db.flush()  # Get test.id