The CAA proxy was caching all fetch failures (including 503s and
timeouts) as empty files, treating them as permanent 'no art' misses.
During Internet Archive outages, this meant every album got cached
as having no cover art, and the cache persisted after IA recovered.
Now only 404 responses (no cover art exists) are cached as permanent
misses. 503, timeouts, and other transient errors are not cached,
so the next request retries the fetch.
Also cleared 33 incorrectly cached 0-byte miss files from a
concurrent IA outage.
Add CoverArtProxy that fetches cover art from CAA, caches the image
bytes on disk (~/.local/share/yellowjacket/cover-art-cache/), and
returns base64 data URLs via the GetThumbnail Wails binding.
First load: fetches from CAA (rate-limited), caches to disk.
Subsequent loads: instant from disk cache, no network.
404s: cached as empty files to avoid re-fetching.
Frontend explore-view loads thumbnails async via GetThumbnail()
calls that fire during render. Cached thumbnails appear as data
URLs directly in img src, bypassing the browser's HTTP stack.
Uncached thumbnails fall back to the CAA URL while the proxy
fetches in the background, then re-render with the cached version.
Also stores caa_id and caa_release_mbid in the search index's
extra_json for future direct Internet Archive URL construction.
yonlu