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)

utils/gg/gg.go

@@ -16,3 +16,13 @@ func If[T any](cond bool, v1, v2 T) T {
 	}
 	return v2
 }
+
+// Clone returns a pointer to a fresh copy of *p, or nil if p is nil. Use it to
+// avoid aliasing the pointed-to value when a separate *T is needed.
+func Clone[T any](p *T) *T {
+	if p == nil {
+		return nil
+	}
+	v := *p
+	return &v
+}

utils/gg/gg_test.go

@@ -46,4 +46,25 @@ var _ = Describe("GG", func() {
 			Expect(gg.If(false, 1.1, 2.2)).To(Equal(2.2))
 		})
 	})
+
+	Describe("Clone", func() {
+		It("returns a pointer to a copy of the value", func() {
+			original := 123
+			cloned := gg.Clone(&original)
+			Expect(cloned).To(HaveValue(Equal(123)))
+			Expect(cloned).NotTo(BeIdenticalTo(&original))
+		})
+
+		It("does not alias the original value", func() {
+			original := 123
+			cloned := gg.Clone(&original)
+			original = 456
+			Expect(*cloned).To(Equal(123))
+		})
+
+		It("returns nil when the input is nil", func() {
+			var v *int
+			Expect(gg.Clone(v)).To(BeNil())
+		})
+	})
 })