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/library/remove_tracks.go

@@ -147,6 +147,11 @@ func (l *Library) RemoveFromLibrary(filePaths []string) (*RemovalResult, error)
 			l.logger.Warn("could not delete FTS entry for removed track",
 				"path", row.FilePath, "id", row.ID, "err", err)
 		}
+
+		if err := l.db.DeleteLyricsIndex(row.ID); err != nil {
+			l.logger.Warn("could not delete lyrics index entry for removed track",
+				"path", row.FilePath, "id", row.ID, "err", err)
+		}
 	}
 
 	// Deleting an audio_files row cascades to queue_tracks, so the