fix: an unfinished download is never stored as an attachment, and the temp sweep keeps any file a library row points at

The importer refuses a staging file (.part, .partial, .megatmp.*) at both
entry points, the same way it refuses a sidecar .json, and
discard_after_attach never deletes one: the age-gated temp sweep owns them.
The sweep checks its stale candidates against image and attachment rows and
keeps any a row points at.

A Google Drive fetch's half-written file had been stored as <sha>.part; the
sweep then deleted it by name and left the row with no file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_importer_attach_in_place.py

@@ -410,6 +410,31 @@ def test_attach_in_place_invalid_image_returns_skipped(importer):
     assert result.skip_reason.value == "invalid_image"
 
 
+@pytest.mark.parametrize("name", ["film.mp4abc123_.part", "img.jpg.partial", ".megatmp.Xy12"])
+def test_an_unfinished_download_is_refused_and_left_on_disk(importer, db_sync, name):
+    """A staging file is a download still being written (or abandoned): it is
+    never stored as an attachment, and the importer never deletes it, so a
+    download in progress is not pulled out from under its writer."""
+    from backend.app.models import PostAttachment
+
+    artist = Artist(name="Hal", slug="hal")
+    db_sync.add(artist)
+    db_sync.flush()
+    staging = importer.images_root / "hal" / "patreon" / "post" / name
+    staging.parent.mkdir(parents=True, exist_ok=True)
+    staging.write_bytes(b"half of a video")
+
+    for result in (importer.attach_in_place(staging, artist=artist),
+                   importer.import_one(staging)):
+        assert result.status == "skipped"
+        assert "unfinished download" in result.error
+        assert importer.discard_after_attach(staging, result) is False
+    assert staging.exists()
+    assert db_sync.execute(
+        select(func.count()).where(PostAttachment.original_filename == name)
+    ).scalar() == 0
+
+
 def test_attach_in_place_non_media_routes_to_attachment(importer, db_sync):
     """FC-2d-iii dispatch parity for the download path. A non-media file
     (.pdf, .txt etc.) downloaded by gallery-dl must become a PostAttachment,