feat(plugins): add lyrics provider plugin capability (#5126)

* feat(plugins): add lyrics provider plugin capability

Refactor the lyrics system from a static function to an interface-based
service that supports WASM plugin providers. Plugins listed in the
LyricsPriority config (alongside "embedded" and file extensions) are
now resolved through the plugin system.

Includes capability definition, Go/Rust PDK, adapter, Wire integration,
and tests for plugin fallback behavior.

* test(plugins): add lyrics capability integration test with test plugin

* fix(plugins): default lyrics language to 'xxx' when plugin omits it

Per the OpenSubsonic spec, the server must return 'und' or 'xxx' when
the lyrics language is unknown. The lyrics plugin adapter was passing
an empty string through when a plugin didn't provide a language value.
This defaults the language to 'xxx', consistent with all other callers
of model.ToLyrics() in the codebase.

* refactor(plugins): rename lyrics import to improve clarity

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(lyrics): update TrackInfo description for clarity

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(lyrics): enhance lyrics plugin handling and case sensitivity

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(plugins): update payload type to string with byte format for task data

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

server/subsonic/api.go

@@ -14,6 +14,7 @@ import (
 	"github.com/navidrome/navidrome/core"
 	"github.com/navidrome/navidrome/core/artwork"
 	"github.com/navidrome/navidrome/core/external"
+	lyricssvc "github.com/navidrome/navidrome/core/lyrics"
 	"github.com/navidrome/navidrome/core/metrics"
 	"github.com/navidrome/navidrome/core/playback"
 	playlistsvc "github.com/navidrome/navidrome/core/playlists"
@@ -48,12 +49,13 @@ type Router struct {
 	share     core.Share
 	playback  playback.PlaybackServer
 	metrics   metrics.Metrics
+	lyrics    lyricssvc.Lyrics
 }
 
 func New(ds model.DataStore, artwork artwork.Artwork, streamer core.MediaStreamer, archiver core.Archiver,
 	players core.Players, provider external.Provider, scanner model.Scanner, broker events.Broker,
 	playlists playlistsvc.Playlists, scrobbler scrobbler.PlayTracker, share core.Share, playback playback.PlaybackServer,
-	metrics metrics.Metrics,
+	metrics metrics.Metrics, lyrics lyricssvc.Lyrics,
 ) *Router {
 	r := &Router{
 		ds:        ds,
@@ -69,6 +71,7 @@ func New(ds model.DataStore, artwork artwork.Artwork, streamer core.MediaStreame
 		share:     share,
 		playback:  playback,
 		metrics:   metrics,
+		lyrics:    lyrics,
 	}
 	r.Handler = r.routes()
 	return r