feat(backend): enforce campaign scheduling, add reviewer queue filter, tactic order, duplicate-test counts

- start_execution now blocks a test from starting before its campaign's
  start_date, closing a gap where the field was persisted but never
  enforced
- GET /tests gains a reviewer_id "my reviews" filter for the red_review/
  blue_review lead-gate stage, distinct from pending_validation_side
  (which only ever covered the later in_review stage and ignored
  assignment entirely)
- get_coverage_by_tactic now returns all 14 MITRE tactics in canonical
  kill-chain order instead of an alphabetical, partial list — the
  regular Dashboard and Executive Dashboard both consume this endpoint
  and previously disagreed on order/completeness
- test-template listing now includes existing_test_count per technique
  so the catalog can warn before creating a likely-duplicate test

backend/app/routers/tests.py

@@ -218,6 +218,10 @@ def list_tests(
     pending_validation_side: Optional[str] = Query(
         None, description="Filter in_review tests pending validation on 'red' or 'blue' side"
     ),
+    reviewer_id: Optional[uuid.UUID] = Query(
+        None,
+        description="\"My reviews\" queue — filter red_review/blue_review tests assigned to this reviewer",
+    ),
     not_in_any_campaign: bool = Query(
         False, description="Only return tests not linked to any campaign"
     ),
@@ -259,6 +263,7 @@ def list_tests(
         created_by=created_by,
         # Keyword argument: pending_validation_side
         pending_validation_side=pending_validation_side,
+        reviewer_id=reviewer_id,
         not_in_any_campaign=not_in_any_campaign,
         offset=offset,
         # Keyword argument: limit