Follow-up to the preferred-art feature. Real test runs showed a source could
win on priority while handing back a small cover: Cover Art Archive is
volunteer-uploaded with no size floor, so CAA-first gave a 599x531 (Taylor
Swift) and a 600x600 (Kendrick GNX) -- front-1200 only caps the max, so a
~600px upload stays ~600px -- and Deezer/iTunes lower in the order never got a
turn.
Fix:
- Minimum-resolution guard: artwork._min_size_art_validator builds the
resolver's validate hook -- it fetches each candidate, caches the bytes (so
the winner isn't fetched twice), and accepts art only when its shortest side
>= metadata_enhancement.min_art_size (default 1000px; 0 disables). Art that's
too small is a miss, so the resolver falls through to the next source instead
of winning on priority. Unmeasurable images are accepted (don't over-reject;
fallback is still today's art). Wired into both embed_album_art_metadata and
download_cover_art.
- iTunes art upgraded to /3000x3000bb/ (was the 600px default) so it
contributes high-res when it wins.
- select_preferred_art_url gains a validate passthrough to the resolver.
- config default metadata_enhancement.min_art_size: 1000.
Effect: with an order like caa > deezer > spotify > itunes, a ~600px CAA upload
is now skipped and Deezer's ~1900px wins -- consistent big art. (Spotify art
often maxes ~640px, so it's skipped at the 1000 floor in favor of bigger
sources; lower min_art_size to ~640 to allow it.)
Tests: tests/metadata/test_art_min_size.py (6 -- incl. the real 599x531 and
600x600 cases, shortest-side logic, unmeasurable-accept, no-bytes-reject,
0-disables) + iTunes max-res upgrade test. Full metadata suite green (617).
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