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/tests/test_campaign_schedule_gate.py

@@ -0,0 +1,76 @@
+"""Tests for the campaign start_date scheduling gate on test execution.
+
+Block 3 fix: `Campaign.start_date` was persisted and displayed but never
+actually enforced — a test belonging to a campaign scheduled for the future
+could still be started immediately. `start_execution` now blocks this.
+"""
+
+from datetime import datetime, timedelta
+
+import pytest
+
+from app.domain.errors import BusinessRuleViolation
+from app.models.campaign import Campaign, CampaignTest
+from app.models.enums import TestState
+from app.models.technique import Technique
+from app.models.test import Test
+from app.services.test_workflow_service import start_execution
+
+
+def _seed_test_in_campaign(db, owner_id, start_date):
+    tech = Technique(mitre_id="T1059.003", name="Windows Command Shell", tactic="execution", platforms=["windows"])
+    db.add(tech)
+    db.flush()
+
+    campaign = Campaign(name="Scheduled Campaign", type="custom", status="active", created_by=owner_id, start_date=start_date)
+    db.add(campaign)
+    db.flush()
+
+    test = Test(technique_id=tech.id, name="Scheduled test", state=TestState.draft, created_by=owner_id)
+    db.add(test)
+    db.flush()
+
+    db.add(CampaignTest(campaign_id=campaign.id, test_id=test.id, order_index=0))
+    db.commit()
+    db.refresh(test)
+    return test
+
+
+def test_start_execution_blocked_before_campaign_start_date(db, red_tech_user):
+    future = datetime.utcnow() + timedelta(days=7)
+    test = _seed_test_in_campaign(db, red_tech_user.id, future)
+
+    with pytest.raises(BusinessRuleViolation):
+        start_execution(db, test, red_tech_user)
+
+
+def test_start_execution_allowed_after_campaign_start_date(db, red_tech_user):
+    past = datetime.utcnow() - timedelta(days=1)
+    test = _seed_test_in_campaign(db, red_tech_user.id, past)
+
+    result = start_execution(db, test, red_tech_user)
+
+    assert result.state == TestState.red_executing
+
+
+def test_start_execution_allowed_for_standalone_test(db, red_tech_user):
+    """A test with no campaign at all is never gated by this check."""
+    tech = Technique(mitre_id="T1059.004", name="Unix Shell", tactic="execution", platforms=["linux"])
+    db.add(tech)
+    db.flush()
+    test = Test(technique_id=tech.id, name="Standalone test", state=TestState.draft, created_by=red_tech_user.id)
+    db.add(test)
+    db.commit()
+    db.refresh(test)
+
+    result = start_execution(db, test, red_tech_user)
+
+    assert result.state == TestState.red_executing
+
+
+def test_start_execution_allowed_when_campaign_has_no_start_date(db, red_tech_user):
+    test = _seed_test_in_campaign(db, red_tech_user.id, None)
+
+    result = start_execution(db, test, red_tech_user)
+
+    assert result.state == TestState.red_executing