Add YouTube ID detection and metadata lookup via yt-dlp

- Extract 11-char YouTube video IDs from audio filenames
- Fetch title, uploader, chapters via yt-dlp (--dump-json)
- Use chapters as tracklist when no .txt tracklist is available
- Store yt_title / yt_uploader in AlbumHints for LLM prompt context
- Fall back to YouTube video title as track title for single-file albums

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models.py

@@ -50,6 +50,8 @@ class AlbumHints:
     tracklist_text: Optional[str] = None   # merged text from all tracklist files
     cover_images: List[Path] = field(default_factory=list)
     tracks: List[TrackHints] = field(default_factory=list)
+    yt_title: Optional[str] = None         # YouTube video title (if found)
+    yt_uploader: Optional[str] = None      # YouTube channel/uploader name
 
 
 @dataclass