feat(dlm): Phase 8 — Detection Lifecycle Management [FASE-8]

Tasks 8.1-8.5:

Models (8.1):
- DetectionAsset: SIEM/EDR/Sigma rule assets with auto-hash
- DetectionTechniqueMapping: N:M asset ↔ technique coverage
- DetectionValidation: immutable validation records with expiry
- TechniqueConfidenceScore: computed multi-factor confidence
- InfrastructureChangeLog: infra changes that invalidate detections
- DecayPolicy: configurable freshness thresholds per platform/tactic

Services (8.2, 8.3):
- detection_asset_service: CRUD + SHA-256 rule hashing + auto-
  invalidation on rule/infra changes
- decay_engine_service: daily decay engine — expires stale validations,
  recalculates confidence (recency/coverage/health/diversity factors),
  processes infrastructure change propagation

Router (8.4): 15 endpoints under /api/v1/detection-lifecycle:
  assets CRUD, technique mappings, validations, confidence scores,
  infrastructure changes, decay trigger, executive dashboard

Scheduler (8.3): decay engine runs daily at 02:00
Seed (8.5): default policy (90/180/365d) + strict initial-access policy
Migration: b034dlm (6 tables, 11 indexes)

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

backend/app/jobs/mitre_sync_job.py

@@ -195,6 +195,20 @@ def _run_stale_detection() -> None:
         db.close()
 
 
+def _run_decay_engine() -> None:
+    """Execute the decay engine inside its own DB session."""
+    logger.info("Scheduled decay engine job starting...")
+    db = SessionLocal()
+    try:
+        from app.services.decay_engine_service import run_decay_engine
+        results = run_decay_engine(db)
+        logger.info("Decay engine job finished — %s", results)
+    except Exception:
+        logger.exception("Decay engine job failed")
+    finally:
+        db.close()
+
+
 # ---------------------------------------------------------------------------
 # Scheduler bootstrap
 # ---------------------------------------------------------------------------
@@ -292,6 +306,15 @@ def start_scheduler() -> None:
         name="Data sources auto-sync (every 6h)",
         replace_existing=True,
     )
+    scheduler.add_job(
+        _run_decay_engine,
+        trigger="cron",
+        hour=2,
+        minute=0,
+        id="decay_engine",
+        name="Detection decay engine (daily 02:00)",
+        replace_existing=True,
+    )
     scheduler.start()
     logger.info(
         "Background scheduler started — mitre_sync (24h), intel_scan (7d), "