fix(downloads): tighten SourceAdapter resolution + neutral nfo default (review)

- resolve_adapter: identify from the yt-dlp extractor_key only, not the full
  webpage_url (a sibling source's name in a URL path no longer misresolves);
  add youtube-nocookie.com to the YouTube host match so nocookie embeds keep
  the POT/player-client extraction strategy.
- _download: re-resolve the adapter with the post-extraction info dict so the
  .nfo uniqueid type and uploader_url agree on one source (dedups the inline
  resolve too).
- MediaMeta.uniqueid_type defaults to source-neutral "generic" instead of
  "youtube", so an un-updated caller can never falsely label a non-YouTube
  download as youtube.
- Tests: lock extractor-key-only resolution + nocookie host; default nfo type.

backend/tests/test_storage.py

@@ -145,8 +145,10 @@ def test_nfo_uniqueid_type_reflects_the_source():
     from app.downloads.storage import _nfo_xml
 
     base = dict(video_id="v1", title="t", uploader="c", upload_date=None)
-    assert '<uniqueid type="youtube"' in _nfo_xml(MediaMeta(**base))  # default preserved
+    # An un-set caller gets the source-neutral "generic" default, never a false "youtube".
+    assert '<uniqueid type="generic"' in _nfo_xml(MediaMeta(**base))
     assert '<uniqueid type="facebook"' in _nfo_xml(MediaMeta(**base, uniqueid_type="facebook"))
+    assert '<uniqueid type="youtube"' in _nfo_xml(MediaMeta(**base, uniqueid_type="youtube"))
 
 
 def test_sidecars_keep_the_full_basename_when_the_title_has_a_dot(tmp_path: Path):