Merge PR #1031: conservative music metadata enrichment for audio downloads

Adds MusicMetadataPreProcessor (app/music_metadata.py), a pre_process
postprocessor that enriches the info dict using only extractor-owned fields:
track-number/total resolution, album-title fallback, and square-thumbnail
preference. Track numbers and album flow into files through yt-dlp's existing
FFmpegMetadata embedding, so no per-format tag-writing code and no new
dependency. The earlier mutagen writer and its download-failing
PostProcessingError path were dropped per review.

Co-authored-by: jahruz67 <jahruz67@users.noreply.github.com>

app/ytdl.py

@@ -23,6 +23,7 @@ from yt_dlp.postprocessor.common import PostProcessor
 from yt_dlp.utils import STR_FORMAT_RE_TMPL, STR_FORMAT_TYPES
 import bg_tasks
 from dl_formats import get_format, get_opts, AUDIO_FORMATS, merge_ytdl_option_layers
+from music_metadata import MusicMetadataPreProcessor
 from datetime import datetime
 from state_store import AtomicJsonStore, from_json_compatible, read_legacy_shelf, to_json_compatible
 from subscriptions import _entry_id
@@ -625,6 +626,13 @@ class Download:
         )
         if getattr(self.info, 'download_type', '') == 'audio':
             ydl.add_post_processor(_AlbumArtistPostProcessor(ydl), when='pre_process')
+            ydl.add_post_processor(
+                MusicMetadataPreProcessor(
+                    ydl,
+                    source_entry=getattr(self.info, 'entry', None),
+                ),
+                when='pre_process',
+            )
         return ydl
 
     def _download(self):