Add a built-in tag editor for both libraries

A new Tags tab browses either library; songs can be edited one at a time
or several at once (e.g. a whole album, where per-song fields are locked
and differing values are kept unless changed). Review's compare panel
links to the editor for either side.

Tags are read with ffprobe and written with ffmpeg (MP3/ID3, FLAC, M4A,
OGG, Opus). An existing tag is always written under the name the file
already uses, so edits replace values instead of duplicating them (e.g.
raw Vorbis ALBUMARTIST/TRACKNUMBER), totals follow the file's style
("3/12" or a separate TRACKTOTAL), stale alias tags (YEAR next to DATE)
are removed, other tags and cover art are kept, and no encoder tag is
added. Every written file is verified: values must read back and the
decoded audio must be bit-identical.

Hardlinks are respected: a file used by one library is replaced
atomically (old file to quarantine); a file shared with the other
library is updated in place so the link survives, after a verified
backup copy goes to quarantine, with an option to edit only one
library's copy instead. Edits are operations (old -> new values shown in
History) that can be undone; an interrupted in-place edit is rolled back
on startup. Dry run applies. Migration 6 stores operation details.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -71,6 +71,18 @@ anything runs, a preview shows how many songs will be processed and which are sk
 downgrades, approximate matches, duration mismatches and files that move folders. Downgrades are skipped unless you tick a box, and
 replacing master files needs an explicit acknowledgement. Each song then goes through the same checks as a single action.
 
+**Tag editor.** The *Tags* tab browses either library. Click a song to edit its tags (title, artist, album artist, album, track and
+disc numbers with totals, year, genre, composer, comment), or tick several, such as a whole album, to edit the tags they share. It
+works for MP3, FLAC, M4A, OGG and Opus; other tags and cover art are kept. Review's compare panel also has **Slave tags** / **Master
+tags** buttons. Edits are safe with hardlinks:
+
+- A file used by only one library is rewritten to a new file that atomically replaces it; the old file goes to quarantine.
+- A file hardlinked to the other library is updated in place, so both libraries see the new tags and stay linked; a verified backup
+  copy goes to quarantine first. You can instead choose to change only one library's copy, which breaks the link.
+- Every written file is verified: the new values must read back, and the decoded audio must be identical to the original.
+- Each edit is listed in History (old → new values) and can be undone; an in-place edit interrupted by a crash is rolled back on
+  startup. Dry run applies.
+
 **Export.** The Review tab's **Export CSV** button downloads either the current view (filter and search applied) or the entire scan.
 Each row has the match status and type, a recommendation, both paths, where linking would place the file, codec, bitrate, sample rate,
 bit depth and duration for both sides, the quality verdict and any notes. The file opens cleanly in Excel, Numbers and LibreOffice
@@ -149,7 +161,8 @@ Run a scan, check the Review tab, and turn off dry run once the planned actions
 | `PLAYLINK_DATA_DIR` | `/config` (Docker) or `./config` | Where the SQLite database lives. |
 | `PLAYLINK_ALLOWED_ROOTS` | `/data` | Comma-separated folders that libraries and the quarantine must live in. `/` is refused. |
 | `PLAYLINK_SECURE_COOKIES` | `false` | Set to `true` when served over HTTPS. |
-| `PLAYLINK_FFPROBE` | `ffprobe` | Path to ffprobe. Without it, quality comparison is disabled but everything else works. |
+| `PLAYLINK_FFPROBE` | `ffprobe` | Path to ffprobe. Without it, quality comparison and tag editing are disabled. |
+| `PLAYLINK_FFMPEG` | `ffmpeg` | Path to ffmpeg, used by the tag editor. |
 | `PLAYLINK_LOG_LEVEL` | `info` | `debug` for verbose logs. |
 
 **Hide quarantine from Plex** (Settings → Media servers) writes a `.plexignore` into each pair's quarantine folder so Plex does not