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/library.go

@@ -1034,7 +1034,7 @@ func (l *Library) scanInternal(
 				}
 			}
 
-			// Remove from FTS5 search index.
+			// Remove from FTS5 search index and the lyrics index.
 			if err := l.db.DeleteSearchIndex(f.ID); err != nil {
 				l.logger.Warn(
 					"failed to delete FTS entry for orphan",
@@ -1045,6 +1045,16 @@ func (l *Library) scanInternal(
 				metrics.addWarning(path, "orphan", err)
 			}
 
+			if err := l.db.DeleteLyricsIndex(f.ID); err != nil {
+				l.logger.Warn(
+					"failed to delete lyrics index entry for orphan",
+					"id", f.ID,
+					"err", err,
+				)
+
+				metrics.addWarning(path, "orphan", err)
+			}
+
 			removed.Add(1)
 		}