Implement songLyrics extension: getLyricsBySongId with real word/syllable cues

Adds getLyricsBySongId (versions 1 and 2 of the songLyrics extension,
gated by the enhanced param): returns every stored structuredLyrics
entry for a song, including line-level start times for synced lyrics
and, only when a source actually provided word/syllable timing (an
Enhanced LRC sidecar) and enhanced=true was requested, cueLine/cue
data with the precomputed UTF-8 byte offsets. Plain and non-enhanced
synced lyrics never get fabricated cue data.

Removes the now-dead SetSongLyrics writer (superseded by
database.InsertLyrics); SongLyricsPlainText keeps serving the legacy
getLyrics endpoint unchanged.

Adds a scanner integration test proving a sidecar .lrc (with an
Enhanced LRC line) is discovered and indexed during a real scan.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014r98AtcBbevpscWLsfwVvv

internal/scanner/scanner_test.go

@@ -237,3 +237,45 @@ func TestScanFolderRejectsConcurrentScan(t *testing.T) {
 		t.Fatal("expected error scanning a folder that is already being scanned")
 	}
 }
+
+func TestScanFolderIndexesSidecarLRC(t *testing.T) {
+	s, folder, dir := newTestScanner(t)
+	genFile(t, dir, "song1.mp3", "Song One", "Artist One")
+
+	lrc := "[ar:Artist One]\n[00:01.00]Hello world\n[00:02.50]<00:02.50>Second <00:02.90>line\n"
+	if err := os.WriteFile(filepath.Join(dir, "song1.lrc"), []byte(lrc), 0o644); err != nil {
+		t.Fatalf("write sidecar lrc: %v", err)
+	}
+
+	if err := s.ScanFolder(context.Background(), folder.ID); err != nil {
+		t.Fatalf("ScanFolder: %v", err)
+	}
+
+	songs, err := database.ListSongs(s.DB, 10, 0)
+	if err != nil || len(songs) != 1 {
+		t.Fatalf("ListSongs: %v %v", songs, err)
+	}
+
+	entries, err := database.GetSongStructuredLyrics(s.DB, songs[0].ID)
+	if err != nil {
+		t.Fatalf("GetSongStructuredLyrics: %v", err)
+	}
+	var synced *database.StructuredLyricsRow
+	for i := range entries {
+		if entries[i].Synced {
+			synced = &entries[i]
+		}
+	}
+	if synced == nil {
+		t.Fatalf("expected a synced structuredLyrics entry from the sidecar .lrc, got %+v", entries)
+	}
+	if len(synced.Lines) != 2 || synced.Lines[0].Text != "Hello world" {
+		t.Fatalf("lines = %+v", synced.Lines)
+	}
+	if synced.Lines[0].StartMs == nil || *synced.Lines[0].StartMs != 1000 {
+		t.Errorf("line[0].StartMs = %v, want 1000", synced.Lines[0].StartMs)
+	}
+	if len(synced.Lines[1].Words) != 2 {
+		t.Fatalf("expected word-level cues from Enhanced LRC line, got %+v", synced.Lines[1])
+	}
+}