feat(subsonic): add structured sidecar lyrics support with OpenSubsonic v2 karaoke cues and agent layers (#5076)

Expand backend lyrics support with richer sidecar formats and upgrade the
OpenSubsonic songLyrics implementation to the version 2 structured karaoke
contract, while preserving version 1 behavior by default.

Sidecar formats and parsing:
- Add a TTML parser (core/lyrics/ttml.go): clock time, offset time, bare
  decimal seconds, nested timing contexts, and token-level <span> timing for
  word/syllable karaoke. Parses Apple Music-style metadata tracks (translation
  and pronunciation/transliteration) and agent metadata into per-track agents[]
  plus per-cue-line agentId. Hydrates missing line timing from cue timing.
- Add an SRT parser (core/lyrics/srt.go).
- Add a LRCLIB Lyricsfile (.yaml/.yml) parser (model/lyricsfile.go): maps
  per-word lines[].words[] to cues with inclusive UTF-8 byte offsets and
  attributes overlapping lines to synthetic voice agents so parallel vocals
  split correctly in the enhanced response.
- Extend LRC parsing for Enhanced LRC inline <mm:ss.xx> word-timing markers.
- Add UTF-8 BOM and UTF-16 LE support for TTML/LRC sidecars.
- Parse the above formats from embedded tags as well as sidecar files.

Source resolution:
- Default lyricspriority is now
  ".ttml,.yaml,.yml,.elrc,.lrc,.srt,.txt,embedded" so the new formats are
  discoverable without manual configuration.
- Preserve configured source priority across duplicate media-file candidates
  instead of only checking the first DB match, so higher-priority sidecar
  lyrics on older duplicates can still win.
- Raise the embedded-lyrics tag maxLength to 1 MB to fit word-timed
  TTML/Enhanced-LRC karaoke for a full song.

OpenSubsonic songLyrics v2:
- Advertise songLyrics versions [1, 2].
- With enhanced=true, getLyricsBySongId may return structuredLyrics.kind
  (main/translation/pronunciation), cueLine[] line-level karaoke groupings,
  cueLine.cue[] timed words/syllables with required UTF-8 byteStart/byteEnd,
  reusable structuredLyrics.agents[], and cueLine.agentId references.
- Without enhanced=true, the response stays v1-compatible: no kind, no cueLine,
  no agents, no non-main tracks; the existing line[] payload is always
  populated so legacy clients keep working.

Contract details:
- cueLine is emitted only for synced lyrics with cue data.
- Within a cueLine, cue.end is normalized all-or-none and overlaps are removed;
  overlaps across separate cueLines remain valid for parallel vocal layers.
- Missing cue end-times are filled from the next cue or the parent line.
- When cueLines share an index, the one whose agent has role "main" is first.
- LyricCue.Value is serialized as XML chardata; cues with nil start are skipped
  rather than serialized as 0.

Refactoring:
- Move pure format parsers into model/ (lyrics.go, lyrics_ttml.go,
  lyrics_srt.go, lyrics_embedded.go, lyricsfile.go) and extract Subsonic
  response building into server/subsonic/lyrics.go.
- Centralize lyric-kind constants and add Lyrics.EffectiveKind/IsMainKind.
- Add gg.Clone helper.

Spec references:
  https://github.com/opensubsonic/open-subsonic-api/discussions/213
  https://github.com/opensubsonic/open-subsonic-api/pull/218 (songLyrics v2)
  https://github.com/opensubsonic/open-subsonic-api/pull/228 (cue byte offsets)

plugins/manager.go

@@ -241,7 +241,7 @@ func (m *Manager) LoadScrobbler(name string) (scrobbler.Scrobbler, bool) {
 	return loadPlugin(m, name, CapabilityScrobbler, newScrobblerPlugin)
 }
 
-func (m *Manager) LoadLyricsProvider(name string) (lyrics.Lyrics, bool) {
+func (m *Manager) LoadLyricsProvider(name string) (lyrics.Provider, bool) {
 	return loadPlugin(m, name, CapabilityLyrics, newLyricsPlugin)
 }