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/detection_asset_service.py

@@ -2,7 +2,7 @@
 
 import hashlib
 import logging
-from datetime import datetime, timezone
+from datetime import datetime
 from typing import Optional
 from uuid import UUID
 
@@ -25,7 +25,7 @@ def _compute_rule_hash(content: str) -> str:
 
 
 def _now() -> datetime:
-    return datetime.now(timezone.utc)
+    return datetime.utcnow()
 
 
 def create_detection_asset(db: Session, data: dict, user_id: UUID) -> DetectionAsset: