fix(maintenance): bound lyrics search and clicks, clear queue source

Three unbounded or stale surfaces, each small on its own:

- lyrics_index rows were never pruned on track removal, so the FTS index
  grew forever. Delete the entry where the library search FTS entry is
  already deleted, on the orphan and RemoveFromLibrary paths.
- search_clicks had no ceiling; age out ranking rows after a retention
  window via a daily janitor job.
- queue.source_* kept a "Playing from X" label after its playlist was
  deleted. Drop the source when the queue's own playlist goes, wired
  through a playlist-service hook like Library.SetRemovalHooks.

Closes #249

backend/database/lyrics_search.go

@@ -151,16 +151,28 @@ func (d *DB) SetLyrics(audioFileID int64, lyrics, source, recordingMBID string)
 	return d.upsertLyricsIndex(audioFileID, lyrics)
 }
 
-// upsertLyricsIndex refreshes a single file's entry in the contentless
-// lyrics_index.  contentless_delete=1 makes the DELETE valid; an empty
-// lyrics string leaves the row deleted.
-func (d *DB) upsertLyricsIndex(audioFileID int64, lyrics string) error {
+// DeleteLyricsIndex removes one file's entry from the contentless
+// lyrics_index.  It is called wherever a file row is deleted — the
+// `lyrics` table cascades with its file, but the FTS entry does not and
+// would otherwise accumulate for the life of the install (#249).
+func (d *DB) DeleteLyricsIndex(audioFileID int64) error {
 	if _, err := d.db.ExecContext(d.Ctx,
 		"DELETE FROM lyrics_index WHERE rowid = ?", audioFileID,
 	); err != nil {
 		return fmt.Errorf("could not delete lyrics_index row: %w", err)
 	}
 
+	return nil
+}
+
+// upsertLyricsIndex refreshes a single file's entry in the contentless
+// lyrics_index.  contentless_delete=1 makes the DELETE valid; an empty
+// lyrics string leaves the row deleted.
+func (d *DB) upsertLyricsIndex(audioFileID int64, lyrics string) error {
+	if err := d.DeleteLyricsIndex(audioFileID); err != nil {
+		return err
+	}
+
 	if strings.TrimSpace(lyrics) == "" {
 		return nil
 	}