fix(assets): refresh loader_path when re-ingesting an existing reference

upsert_reference only wrote loader_path on the INSERT branch, so
re-ingesting an existing reference (an output overwritten in place, or a
file re-registered after its loader_path derivation changed) kept the
stale or NULL value forever. Write it on the UPDATE branch too, with a
null-safe change guard so a loader_path difference alone is enough to
trigger the update, and identical values stay a no-op.

app/assets/database/queries/asset_reference.py

@@ -688,13 +688,14 @@ def upsert_reference(
                 AssetReference.asset_id != asset_id,
                 AssetReference.mtime_ns.is_(None),
                 AssetReference.mtime_ns != int(mtime_ns),
+                AssetReference.loader_path.is_distinct_from(loader_path),
                 AssetReference.is_missing == True,  # noqa: E712
                 AssetReference.deleted_at.isnot(None),
             )
         )
         .values(
-            asset_id=asset_id, mtime_ns=int(mtime_ns), is_missing=False,
-            deleted_at=None, updated_at=now,
+            asset_id=asset_id, mtime_ns=int(mtime_ns), loader_path=loader_path,
+            is_missing=False, deleted_at=None, updated_at=now,
         )
     )
     res2 = session.execute(upd)

tests-unit/assets_test/queries/test_cache_state.py

@@ -176,6 +176,39 @@ class TestUpsertReference:
         ref = session.query(AssetReference).filter_by(file_path=file_path).one()
         assert ref.mtime_ns == final_mtime
 
+    def test_upsert_refreshes_loader_path_on_existing_reference(self, session: Session):
+        """Re-ingesting an existing reference writes the loader_path computed
+        by that ingest, healing NULL or stale values even when nothing else
+        about the row changed."""
+        asset = _make_asset(session, "hash1")
+        file_path = "/models/checkpoints/sub/model.safetensors"
+
+        upsert_reference(
+            session, asset_id=asset.id, file_path=file_path, name="model",
+            mtime_ns=100, loader_path=None,
+        )
+        session.commit()
+
+        created, updated = upsert_reference(
+            session, asset_id=asset.id, file_path=file_path, name="model",
+            mtime_ns=100, loader_path="sub/model.safetensors",
+        )
+        session.commit()
+
+        assert created is False
+        assert updated is True
+        ref = session.query(AssetReference).filter_by(file_path=file_path).one()
+        assert ref.loader_path == "sub/model.safetensors"
+
+        # Identical loader_path is a no-op, not a spurious update.
+        created, updated = upsert_reference(
+            session, asset_id=asset.id, file_path=file_path, name="model",
+            mtime_ns=100, loader_path="sub/model.safetensors",
+        )
+        session.commit()
+        assert created is False
+        assert updated is False
+
     def test_upsert_restores_missing_reference(self, session: Session):
         """Upserting a reference that was marked missing should restore it."""
         asset = _make_asset(session, "hash1")