music indexer: index track lyrics (.lrc/.txt sidecars + embedded)

Each track's lyrics are resolved and cached at cache/<rel>/lyrics/<file>.<lrc|txt>,
with the format recorded as `lyrics: 'lrc'|'txt'` on the meta.tracks entry.

Precedence: external "<base>.lrc" > external "<base>.txt" > embedded tag
(lyrics / lyrics-<lang> / unsyncedlyrics — ffprobe now reads all format tags).
Content that contains [mm:ss] lines is stored as lrc even from a .txt/embedded
source. Only track-matching sidecars affect the version signature (a stray
notes.txt is ignored). Lyrics dir is wiped+regenerated per rebuild; new
`lyricsIndexed` counter.

Served by GET /api/music/lyrics?path=<rel>&file=<track> (text/plain +
X-Lyrics-Format header, ETag=<v>, 304, 404 when none).

Verified end-to-end: external .lrc wins over embedded; embedded → plain txt;
unmatched .txt ignored. MUSIC_API.md documents the field + endpoint.

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

MUSIC_API.md

@@ -96,7 +96,8 @@ Returns the album's `meta.json`. Sends `ETag: <v>`; a request with `If-None-Matc
       "album": "Back in Black",
       "track": "1",
       "year": "1980",
-      "durationSec": 312
+      "durationSec": 312,
+      "lyrics": "lrc"                       // present if lyrics exist: "lrc" = synced, "txt" = plain (see §2.3.2)
     }
     // …
   ],
@@ -134,6 +135,19 @@ A compressed frame grab for a video (≤600px, same treatment as covers), taken
 the video's filename within `<rel>` (URL-encode it). Sends `ETag: <v>`; `If-None-Match: <v>` → `304`; `404`
 when the video has no poster. Only request it when that video's `meta.videos[]` entry has a `poster` field.
 
+#### 2.3.2 Lyrics
+
+```
+GET /api/music/lyrics?path=<rel>&file=<track filename>
+```
+Plain-text body of the track's lyrics; the `X-Lyrics-Format` header is `lrc` (synced, `[mm:ss.xx]`-timestamped)
+or `txt` (plain). Sends `ETag: <v>`; `If-None-Match: <v>` → `304`; `404` when the track has no lyrics. Only
+request it when that track's `meta.tracks[]` entry has a `lyrics` field (`"lrc"`/`"txt"`).
+
+Sources, in precedence order (indexed at build time): an external **`<track basename>.lrc`** > external
+**`<track basename>.txt`** > **embedded** lyrics in the audio tags (`lyrics` / `lyrics-<lang>` /
+`unsyncedlyrics`). A `.txt` (or embedded) whose text actually contains `[mm:ss]` lines is served as `lrc`.
+
 ### 2.4 Discography (artist album grouping)
 
 For artist folders (manifest entry with `"disco": true`), this returns a map of **album folder → release
@@ -184,7 +198,7 @@ GET  /api/music/reindex/status   → IndexStatus snapshot
   "running": true,
   "startedAt": 1785034701973, "finishedAt": null,
   "foldersScanned": 45, "albumsBuilt": 12, "albumsSkipped": 3,
-  "tracksIndexed": 320, "videosIndexed": 4, "coversSaved": 12, "postersSaved": 4, "discographies": 3,
+  "tracksIndexed": 320, "videosIndexed": 4, "coversSaved": 12, "postersSaved": 4, "lyricsIndexed": 45, "discographies": 3,
   "currentPath": "Albums/AC-DC/[1980] Back in Black",
   "error": null
 }