Fix API/frontend type mismatch for dashboard

- Backend: Change 'id' to 'artifact_id' in most_referenced_artifacts response
- Backend: Add content_type field to referenced artifacts
- Frontend: Add orphaned_size_bytes to Stats interface
- Frontend: Add missing fields to DeduplicationStats interface

backend/app/routes.py

@@ -2436,11 +2436,12 @@ def get_deduplication_stats(
 
     most_referenced = [
         {
-            "id": a.id,
+            "artifact_id": a.id,
             "ref_count": a.ref_count,
             "size": a.size,
             "storage_saved": a.size * (a.ref_count - 1),
             "original_name": a.original_name,
+            "content_type": a.content_type,
         }
         for a in top_artifacts
     ]