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/seed_decay_policies.py

@@ -0,0 +1,39 @@
+"""Seed default decay policies."""
+from datetime import datetime
+from sqlalchemy.orm import Session
+from app.models.decay_policy import DecayPolicy
+
+
+def seed_decay_policies(db: Session) -> None:
+    existing = db.query(DecayPolicy).filter(DecayPolicy.is_default == True).first()
+    if existing:
+        return
+
+    now = datetime.utcnow()
+    default_policy = DecayPolicy(
+        name="Default Decay Policy",
+        description="Standard: Fresh 90d, Aging 91-180d, Stale 181-365d.",
+        fresh_days=90, aging_days=180, stale_days=365,
+        default_validity_days=180, silent_threshold_days=30,
+        noisy_threshold_daily=100,
+        recency_weight=0.30, coverage_weight=0.30,
+        health_weight=0.25, diversity_weight=0.15,
+        is_default=True, is_active=True,
+        created_at=now, updated_at=now,
+    )
+    db.add(default_policy)
+
+    critical_policy = DecayPolicy(
+        name="Critical Techniques Policy",
+        description="Stricter: Fresh 60d, Aging 90d, Stale 180d.",
+        applies_to_tactic="initial-access",
+        fresh_days=60, aging_days=90, stale_days=180,
+        default_validity_days=90, silent_threshold_days=14,
+        noisy_threshold_daily=50,
+        recency_weight=0.35, coverage_weight=0.30,
+        health_weight=0.25, diversity_weight=0.10,
+        is_default=False, is_active=True,
+        created_at=now, updated_at=now,
+    )
+    db.add(critical_policy)
+    db.commit()