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/library/library-client.tsx

@@ -2,6 +2,7 @@
 
 import { useEffect, useMemo, useState } from "react";
 import { PageHead } from "../_ui/page-head";
+import { SectionHeader } from "../_ui/section-header";
 import { CoverArt } from "../_ui/cover-art";
 import { AlbumModal } from "../_ui/album-modal";
 
@@ -42,14 +43,17 @@ export function LibraryClient() {
           <span>Filter</span>
           <input aria-label="filter library" placeholder="Artist or album…" value={q} onChange={(e) => setQ(e.target.value)} />
         </label>
-        <span className="rmeta">
-          {shown.length} of {albums.length} albums
-        </span>
       </form>
 
       {albums.length === 0 ? (
         <p className="empty">Nothing pressed yet. Scan your library in Settings, or queue a release from The Floor.</p>
       ) : (
+        <SectionHeader
+          title="Pressings"
+          note={q ? `${shown.length} of ${albums.length} albums` : `${albums.length} albums`}
+        />
+      )}
+      {albums.length > 0 ? (
         <div className="album-grid">
           {shown.map((a) => (
             <button key={a.id} className="album-card" onClick={() => setOpen(a)} aria-label={`Open ${a.album}`}>
@@ -64,7 +68,7 @@ export function LibraryClient() {
             </button>
           ))}
         </div>
-      )}
+      ) : null}
 
       {open ? (
         <AlbumModal