Merge branch 'fix/249-unbounded-growth' into batch/248-249

Both branches add to the same two registries, so the conflicts are
between the two fixes rather than with main:

- backend/app.go: both register a janitor job.  Both are registered.
- backend/maintenance/sweeps.go: both append a job at the end of the
  file.  Both are kept, each with its own closing tail.
- backend/maintenance/maintenance_test.go: both append a test.  Both are
  kept as separate functions.
- backend/library/library.go: #249's orphan-path lyrics delete was
  written against the loop variable before #250 renamed it, so its
  `audioFile.ID` no longer exists in that function.  Adapted to `f.ID`.

Closes #248
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
 	}