fix(decay-engine): strip tzinfo from validated_at before datetime arithmetic

The previous fix changed _now() to return naive UTC, but the code still
called .replace(tzinfo=utc) on most_recent (from DB) before subtracting.
This caused "can't subtract offset-naive and offset-aware datetimes".
Now we strip tzinfo if present, keeping everything naive UTC consistently.

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

backend/app/services/decay_engine_service.py

@@ -81,10 +81,9 @@ def calculate_confidence_for_technique(db: Session, technique_id: UUID) -> Optio
     last_validated = None
     if valid_validations:
         most_recent = max(v.validated_at for v in valid_validations)
-        # Make timezone-aware if needed
-        if most_recent.tzinfo is None:
-            from datetime import timezone as _tz
-            most_recent = most_recent.replace(tzinfo=_tz.utc)
+        # Strip tzinfo if present so arithmetic stays consistent with naive UTC
+        if most_recent.tzinfo is not None:
+            most_recent = most_recent.replace(tzinfo=None)
         last_validated = most_recent
         days_since = (now - most_recent).days
         if days_since <= policy.fresh_days: