feat: --retag-from-path to recover tags damaged by a prior --repair

Offline re-tag of artist/title from the artist folder + filename: strips
(Official Video)/(Lyrics)-style decorations and trailing [id], and treats an
'Artist - Title' filename as authoritative (recovering the real artist for
music videos filed under a channel name). Overwrites artist/title only; leaves
album/year. Honors --dry-run.

Refactors the source-folder walk into _iter_source_files, shared by --repair
and --retag-from-path.

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

README.md

@@ -95,6 +95,7 @@ export LIDARR_API_KEY="your-lidarr-api-key"
 | `-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). |
+| `--retag-from-path` | Offline: re-tag artist/title from folder + filename (see below). |
 | `--debug` | Verbose output. |
 
 ### Examples
@@ -144,6 +145,17 @@ It re-queries the source over the network, so run it occasionally, not constantl
 ./musicfetch --repair -o /media/music
 ```
 
+**`--retag-from-path`** is an offline companion: it derives **artist** and **title** purely
+from the folder name + filename (stripping `(Official Video)` / `(Lyrics)`-style decorations,
+and treating an `Artist - Title` filename correctly), with no network. Use it to undo bad
+tags — e.g. titles/artists clobbered by an earlier `--repair` on music videos. It overwrites
+artist/title and leaves album/year alone.
+
+```bash
+./musicfetch --retag-from-path -d            # preview
+./musicfetch --retag-from-path -o /media/music
+```
+
 ### 📁 Output Structure
 
 ```text