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/maintenance/maintenance_test.go

@@ -666,3 +666,52 @@ func TestExpiredHTTPCacheJob_TrimsToBudget(t *testing.T) {
 		t.Errorf("kept %q, want the longest-lived row", kept)
 	}
 }
+
+// TestStaleSearchClicksJob deletes only the clicks old enough to have
+// left the retention window (#249).
+func TestStaleSearchClicksJob(t *testing.T) {
+	t.Parallel()
+
+	db := database.NewTestDB(t)
+
+	count := func(mbid string) int {
+		t.Helper()
+
+		var n int
+		if err := db.QueryRowWriter(
+			"SELECT COUNT(*) FROM search_clicks WHERE entity_mbid = ?", mbid,
+		).Scan(&n); err != nil {
+			t.Fatalf("count %s: %v", mbid, err)
+		}
+
+		return n
+	}
+
+	seed := func(query, mbid, lastClicked string) {
+		t.Helper()
+
+		if _, err := db.ExecContext(
+			`INSERT INTO search_clicks
+			   (query, entity_mbid, entity_type, click_count, last_clicked)
+			 VALUES (?, ?, 'recording', 1, ?)`,
+			query, mbid, lastClicked,
+		); err != nil {
+			t.Fatalf("seed search_clicks: %v", err)
+		}
+	}
+
+	seed("tide", "aaaa", "2024-01-01 00:00:00") // stale
+	seed("tide", "bbbb", "2999-01-01 00:00:00") // recent
+
+	if _, err := StaleSearchClicksJob(db).Run(context.Background()); err != nil {
+		t.Fatalf("run job: %v", err)
+	}
+
+	if n := count("bbbb"); n != 1 {
+		t.Errorf("recent click was deleted: %d rows, want 1", n)
+	}
+
+	if n := count("aaaa"); n != 0 {
+		t.Errorf("stale click survived: %d rows, want 0", n)
+	}
+}