fix(detection-lifecycle): fix timezone naive/aware mismatch and duplicate technique mapping

- Replace datetime.now(timezone.utc) with datetime.utcnow() in _now() across
  all three Phase 8 files to match DB DateTime column type (naive UTC)
- Guard POST /assets/{id}/techniques/{tid} against duplicate mappings:
  if mapping already exists, update coverage_type/confidence_level instead
  of inserting a duplicate row

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

backend/app/services/decay_engine_service.py

@@ -1,7 +1,7 @@
 """Decay Engine — calculates confidence scores and expires validations."""
 
 import logging
-from datetime import datetime, timezone
+from datetime import datetime
 from typing import Optional
 from uuid import UUID
 
@@ -20,7 +20,7 @@ logger = logging.getLogger(__name__)
 
 
 def _now() -> datetime:
-    return datetime.now(timezone.utc)
+    return datetime.utcnow()
 
 
 def get_applicable_policy(db: Session, platform: Optional[str] = None, asset_type: Optional[str] = None, tactic: Optional[str] = None) -> DecayPolicy: