feat: --repair flag to re-tag existing downloads from source metadata

Walks <root>/<artist>/<source>/ (known yt-dlp source folders only; skips Lidarr
album dirs), re-queries each file's source by the [id] in its filename, and fixes
tags (album/year/artist/title) via mutagen. Honors --dry-run for preview. CLI-only
(not the REST API). Fixes downloads that landed with missing album / wrong year.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -94,6 +94,7 @@ export LIDARR_API_KEY="your-lidarr-api-key"
 | `--yt-only` | Skip Lidarr. |
 | `-o`, `--root PATH` | Output root folder (default `/media/music`). |
 | `--search-all` | Search all albums when adding an artist to Lidarr. |
+| `--repair` | Re-tag existing downloads under `--root` from source metadata (see below). |
 | `--debug` | Verbose output. |
 
 ### Examples
@@ -115,8 +116,26 @@ export LIDARR_API_KEY="your-lidarr-api-key"
 # YouTube only, lossless preferred
 ./musicfetch --yt-only -q flac "Bonobo - Kerala"
 
-# Download by URL (YouTube Music URL preferred for correct art)
+# Download by URL (single track or playlist/set/album, any yt-dlp site)
 ./musicfetch "https://music.youtube.com/watch?v=xxxxxxxxxxx"
+./musicfetch "https://soundcloud.com/artist/sets/my-mix"
+```
+
+### 🔧 Repair existing tags
+
+`--repair` walks `<root>/<artist>/<source>/` (the `youtube`/`soundcloud`/… download
+folders — Lidarr album folders are skipped), re-fetches authoritative metadata for each
+file using the `[id]` in its filename, and fixes tags (album, year, artist, title). Useful
+when downloads landed with missing album or wrong year. It re-queries the source over the
+network, so run it occasionally, not constantly. Requires `mutagen` (a yt-dlp dependency,
+usually already present). CLI-only — not exposed via the REST API.
+
+```bash
+# Preview what would change (writes nothing)
+./musicfetch --repair -d
+
+# Apply fixes under a specific root
+./musicfetch --repair -o /media/music
 ```
 
 ### 📁 Output Structure
@@ -124,8 +143,10 @@ export LIDARR_API_KEY="your-lidarr-api-key"
 ```text
 <root>/
 ├── Artist Name/
-│   ├── Album Name/   (managed by Lidarr)
-│   └── youtube/      (yt-dlp downloads / fallbacks)
+│   ├── Album Name/    (managed by Lidarr)
+│   ├── youtube/       (YouTube / YouTube Music downloads)
+│   ├── soundcloud/    (SoundCloud downloads)
+│   └── <source>/      (one folder per yt-dlp source)
 ```
 
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