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/models/decay_policy.py

@@ -0,0 +1,32 @@
+"""Decay Policy model — configurable detection validity rules."""
+
+import uuid
+from datetime import datetime
+from sqlalchemy import Column, String, Integer, Float, Boolean, DateTime, Text
+from sqlalchemy.dialects.postgresql import UUID
+from app.database import Base
+
+
+class DecayPolicy(Base):
+    __tablename__ = "decay_policies"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    name = Column(String(200), nullable=False)
+    description = Column(Text)
+    applies_to_platform = Column(String(100))
+    applies_to_asset_type = Column(String(50))
+    applies_to_tactic = Column(String(100))
+    fresh_days = Column(Integer, default=90, server_default='90')
+    aging_days = Column(Integer, default=180, server_default='180')
+    stale_days = Column(Integer, default=365, server_default='365')
+    default_validity_days = Column(Integer, default=180, server_default='180')
+    silent_threshold_days = Column(Integer, default=30, server_default='30')
+    noisy_threshold_daily = Column(Integer, default=100, server_default='100')
+    recency_weight = Column(Float, default=0.3, server_default='0.3')
+    coverage_weight = Column(Float, default=0.3, server_default='0.3')
+    health_weight = Column(Float, default=0.25, server_default='0.25')
+    diversity_weight = Column(Float, default=0.15, server_default='0.15')
+    is_default = Column(Boolean, default=False, server_default='false')
+    is_active = Column(Boolean, default=True, server_default='true')
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow)