feat(alerts): Phase 13 — Operational Alert Engine

AlertRule + AlertInstance models (b041alerts migration), 8 pre-seeded system
rules (high_risk x2, stale_technique, coverage_regression, low_coverage,
expiry_wave, new_technique, orphan_spike), evaluation engine with per-rule
cooldown, full alert lifecycle (acknowledge/resolve/dismiss), custom rule CRUD,
and summary endpoint. Rules seeded at app startup.

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

backend/app/models/__init__.py

@@ -43,6 +43,7 @@ from app.models.risk_intelligence import TechniqueRiskProfile
 from app.models.executive_dashboard import PostureSnapshot
 from app.models.api_key import ApiKey
 from app.models.sso_config import SsoConfig
+from app.models.operational_alert import AlertRule, AlertInstance
 
 __all__ = [
     "User", "Technique", "Test", "TestTemplate", "Evidence",
@@ -69,4 +70,6 @@ __all__ = [
     "PostureSnapshot",
     "ApiKey",
     "SsoConfig",
+    "AlertRule",
+    "AlertInstance",
 ]