feat:(server): support reading lyrics from filesystem (#2897)

* simplified lyrics handling

* address initial feedback

* add some trace and error logging

* allow fallback lyrics

* update nit

* restore artist/title filter only

core/lyrics/sources.go

@@ -0,0 +1,51 @@
+package lyrics
+
+import (
+	"context"
+	"errors"
+	"os"
+	"path"
+
+	"github.com/navidrome/navidrome/log"
+	"github.com/navidrome/navidrome/model"
+)
+
+func fromEmbedded(ctx context.Context, mf *model.MediaFile) (model.LyricList, error) {
+	if mf.Lyrics != "" {
+		log.Trace(ctx, "embedded lyrics found in file", "title", mf.Title)
+		return mf.StructuredLyrics()
+	}
+
+	log.Trace(ctx, "no embedded lyrics for file", "path", mf.Title)
+
+	return nil, nil
+}
+
+func fromExternalFile(ctx context.Context, mf *model.MediaFile, suffix string) (model.LyricList, error) {
+	basePath := mf.AbsolutePath()
+	ext := path.Ext(basePath)
+
+	externalLyric := basePath[0:len(basePath)-len(ext)] + suffix
+
+	contents, err := os.ReadFile(externalLyric)
+
+	if errors.Is(err, os.ErrNotExist) {
+		log.Trace(ctx, "no lyrics found at path", "path", externalLyric)
+		return nil, nil
+	} else if err != nil {
+		return nil, err
+	}
+
+	lyrics, err := model.ToLyrics("xxx", string(contents))
+	if err != nil {
+		log.Error(ctx, "error parsing lyric external file", "path", externalLyric, err)
+		return nil, err
+	} else if lyrics == nil {
+		log.Trace(ctx, "empty lyrics from external file", "path", externalLyric)
+		return nil, nil
+	}
+
+	log.Trace(ctx, "retrieved lyrics from external file", "path", externalLyric)
+
+	return model.LyricList{*lyrics}, nil
+}