Add comprehensive ref_count tests and fix resumable upload double-counting bug

- Add tests for cascade deletion ref_count (package/project delete)
- Add tests for tag update ref_count adjustments
- Fix resumable upload bug where ref_count was incremented manually AND by SQL trigger
- ref_count is now exclusively managed by SQL triggers on tag INSERT/DELETE/UPDATE

backend/app/routes.py

@@ -1167,8 +1167,12 @@ def init_resumable_upload(
         db.query(Artifact).filter(Artifact.id == init_request.expected_hash).first()
     )
     if existing_artifact:
-        # File already exists - use atomic increment for ref count
-        _increment_ref_count(db, existing_artifact.id)
+        # File already exists - deduplicated upload
+        # NOTE: ref_count is managed by SQL triggers on tag INSERT/DELETE/UPDATE
+        # We do NOT manually increment here because:
+        # 1. If a tag is provided, _create_or_update_tag will create/update a tag
+        #    and the SQL trigger will handle ref_count
+        # 2. If no tag is provided, ref_count shouldn't change (no new reference)
 
         # Record the upload
         upload = Upload(