feat: real worker status, covers on Recently Pressed, library counter

- Worker stamps a throttled worker.heartbeat; GET /api/worker reports online
  from its updatedAt recency (DB-time); masthead shows a live listening/offline
  dot instead of a static decoration.
- /api/requests enriched with rgMbid; Recently Pressed shows cover thumbs.
- Library album count moved from the filter row into a section header.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/queue.tsx

@@ -14,6 +14,7 @@ type Row = {
   album: string;
   status: string;
   createdAt?: string;
+  rgMbid?: string | null;
   job: { state: string; currentStage: string } | null;
 };
 
@@ -129,7 +130,7 @@ export function Queue() {
         <>
           <SectionHeader title="Recently Pressed" note={`${pressed.length} in library`} />
           <PressedList
-            items={pressed.map((r) => ({ id: r.id, artist: r.artist, album: r.album, matrix: pressedMark(r) }))}
+            items={pressed.map((r) => ({ id: r.id, artist: r.artist, album: r.album, matrix: pressedMark(r), rgMbid: r.rgMbid ?? null }))}
           />
         </>
       ) : null}