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/routers/detection_lifecycle.py

@@ -1,7 +1,7 @@
 """Detection Lifecycle Management router."""
 
 import hashlib
-from datetime import datetime, timezone, timedelta
+from datetime import datetime, timedelta
 from typing import Optional
 from uuid import UUID
 
@@ -28,7 +28,7 @@ router = APIRouter(prefix="/detection-lifecycle", tags=["detection-lifecycle"])
 
 
 def _now() -> datetime:
-    return datetime.now(timezone.utc)
+    return datetime.utcnow()
 
 
 # ── Detection Assets ─────────────────────────────────────────────────────────
@@ -81,6 +81,23 @@ def map_technique(
     db: Session = Depends(get_db),
     user=Depends(get_current_user),
 ):
+    # Validate asset exists
+    asset = db.query(DetectionAsset).filter(DetectionAsset.id == asset_id).first()
+    if not asset:
+        raise EntityNotFoundError("DetectionAsset", str(asset_id))
+
+    # Prevent duplicate mappings
+    existing = db.query(DetectionTechniqueMapping).filter(
+        DetectionTechniqueMapping.detection_asset_id == asset_id,
+        DetectionTechniqueMapping.technique_id == technique_id,
+    ).first()
+    if existing:
+        # Update coverage/confidence on existing mapping instead of duplicating
+        existing.coverage_type = coverage_type
+        existing.confidence_level = confidence_level
+        db.commit()
+        return {"message": "Technique mapping updated", "mapping_id": str(existing.id)}
+
     mapping = DetectionTechniqueMapping(
         detection_asset_id=asset_id, technique_id=technique_id,
         coverage_type=coverage_type, confidence_level=confidence_level,