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/schemas/test_template.py

@@ -102,6 +102,9 @@ class TestTemplateSummary(BaseModel):
     severity: str | None = None
     # Assign is_active = True
     is_active: bool = True
+    # Number of existing Test rows for this template's technique — lets the
+    # catalog UI warn before creating a likely-duplicate test.
+    existing_test_count: int = 0
 
     # Assign model_config = ConfigDict(from_attributes=True)
     model_config = ConfigDict(from_attributes=True)