feat(maintenance): daily janitor for orphaned .part/.partial staging files

Downloads/imports stage into <name>.part / <name>.partial then os.replace() into
place, so a kill mid-write leaves a discardable temp — never a corrupt final.
cleanup_orphaned_temp_files sweeps ones left behind under the images root, only
older than 6h so an in-flight download's staging file is never removed. Daily beat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_maintenance.py

@@ -26,6 +26,28 @@ def _make_batch(session) -> int:
     return batch.id
 
 
+def test_cleanup_orphaned_temp_files_removes_stale_only(tmp_path, monkeypatch):
+    import os
+
+    from backend.app.tasks import maintenance as m
+
+    monkeypatch.setattr(m, "IMAGES_ROOT", tmp_path)
+    stale = tmp_path / "artist" / "img.jpg.part"      # killed download → orphan
+    stale.parent.mkdir(parents=True)
+    stale.write_bytes(b"x")
+    old = datetime.now(UTC).timestamp() - 8 * 3600     # older than the 6h guard
+    os.utime(stale, (old, old))
+    fresh = tmp_path / "in_progress.jpg.partial"       # active download → keep
+    fresh.write_bytes(b"x")
+    keep = tmp_path / "real.jpg"                        # a real image → keep
+    keep.write_bytes(b"x")
+
+    assert m.cleanup_orphaned_temp_files() == 1
+    assert not stale.exists()
+    assert fresh.exists()
+    assert keep.exists()
+
+
 def test_recover_interrupted_only_old(db_sync, monkeypatch):
     batch_id = _make_batch(db_sync)
     now = datetime.now(UTC)