Cover Art Archive URLs are deterministic from the MBID: a GET either
307-redirects to the image or returns 404. The previous adapter fired
`requests.head(timeout=3)` per search result to probe for the image
first. 10 results × 3s worst-case = up to 30s of blocking HEAD calls
before a search returned.
The probe was defensive overhead — the frontend already handles 404 via
`<img onerror>` fallback. Building the URL deterministically and letting
the browser load it lazily collapses the tail latency to the real MB API
calls (artist-search + browse = ~3s at the 1-rps rate limit).
Also prefer release-group scope over per-release scope when both are
available — release-group covers every edition of an album, so the hit
rate is noticeably higher than pinning to a specific regional release.
Removes now-unused `self._art_cache` and the `requests` import.
Broque Thomas