feat(web): cache cover art + dedupe Recently Pressed

- Cover art now loads through GET /api/cover/[rgMbid], which fetches from the
  Cover Art Archive once, caches the image on disk (LYRA_COVER_CACHE, default
  /tmp/lyra-covers), and serves it with a 30-day Cache-Control — so the browser
  stops re-fetching it on every Library / Recently-Pressed render. Missing covers
  get a negative marker (re-checked daily). CoverArt points at the endpoint.
- Recently Pressed dedupes by artist+album (re-requesting/re-downloading an album
  left multiple completed Requests showing the same album repeatedly); newest of
  each is kept.

web 164 tests, tsc + build clean.

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

web/src/app/_ui/cover-art.tsx

@@ -2,9 +2,9 @@
 
 import { useState } from "react";
 
-/** Album art from the Cover Art Archive (keyed by release-group MBID), loaded straight from
- *  their CDN by the browser — no backend, no MB API rate limit. Falls back to a placeholder
- *  when there's no MBID or no art. */
+/** Album art keyed by release-group MBID, served through /api/cover (which caches it on disk +
+ *  sends a long Cache-Control, so it isn't re-fetched from the Cover Art Archive on every
+ *  Library / Recently-Pressed render). Falls back to a placeholder when there's no MBID or art. */
 export function CoverArt({
   rgMbid,
   alt,
@@ -29,7 +29,7 @@ export function CoverArt({
     // eslint-disable-next-line @next/next/no-img-element
     <img
       className={cls}
-      src={`https://coverartarchive.org/release-group/${rgMbid}/front-${size}`}
+      src={`/api/cover/${rgMbid}?size=${size}`}
       alt={alt}
       loading="lazy"
       onError={() => setFailed(true)}