fix(maintenance): bound search_clicks and lyrics_index, clear stale 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/queue/queue_test.go

@@ -535,3 +535,35 @@ func TestCycleRepeat_CyclesThroughModes(t *testing.T) {
 		t.Errorf("after third cycle: got %q, want %q", state.RepeatMode, RepeatOff)
 	}
 }
+
+// TestDropSourceForPlaylist clears the "Playing from" label when the
+// queue's source playlist is deleted, and leaves it alone otherwise
+// (#249).
+func TestDropSourceForPlaylist(t *testing.T) {
+	t.Parallel()
+
+	q, db := setupTestQueue(t)
+	paths := seedAudioFiles(t, db, 2)
+
+	q.SetQueue(paths, 0, false, Source{Type: "playlist", ID: 42, Label: "Road Trip"})
+	q.DropSourceForPlaylist(42)
+
+	if got := q.GetState().Source; got != (Source{}) {
+		t.Errorf("source = %+v, want empty after playlist 42 deleted", got)
+	}
+}
+
+func TestDropSourceForPlaylistIgnoresOtherPlaylists(t *testing.T) {
+	t.Parallel()
+
+	q, db := setupTestQueue(t)
+	paths := seedAudioFiles(t, db, 2)
+
+	source := Source{Type: "smartPlaylist", ID: 42, Label: "Road Trip"}
+	q.SetQueue(paths, 0, false, source)
+	q.DropSourceForPlaylist(7)
+
+	if got := q.GetState().Source; got != source {
+		t.Errorf("source = %+v, want %+v unchanged for a different playlist", got, source)
+	}
+}