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/app/worker.py

@@ -791,6 +791,9 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
         adapter = adapters.resolve_adapter(source_kind, source_ref)
         url = service.source_url(source_kind, source_ref)
         info = _media_info(_extract_with_fallback(job_id, asset_id, url, spec, staging, adapter))
+        # Re-resolve now that yt-dlp's info reveals the true source (a bare/redirected url only
+        # identifies post-extraction) so the .nfo uniqueid type + uploader_url agree on one source.
+        adapter = adapters.resolve_adapter(source_kind, source_ref, info)
 
         produced = _produced_file(info, staging)
         produced = _ensure_browser_playable(job_id, produced, spec, info)
@@ -834,7 +837,7 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
                 asset.acodec = info.get("acodec") if info.get("acodec") not in (None, "none") else None
                 asset.title = meta.title
                 asset.uploader = meta.uploader
-                asset.uploader_url = adapters.resolve_adapter(source_kind, source_ref, info).uploader_url(info)
+                asset.uploader_url = adapter.uploader_url(info)
                 asset.upload_date = meta.upload_date
                 asset.thumbnail_url = meta.thumbnail_url
                 # Record a self-hosted poster for every download: it's the reliable link-preview