fix(cover): stop per-song over-fetch + log failed cover downloads (#944)

* fix(cover): stop per-song cover over-fetch (album/mf-* explosion)

album_has_distinct_disc_covers returned true as soon as two tracks on the same
disc had different cover ids. On Navidrome every song has its own mf-<id>
coverArt, so almost every album was flagged "distinct disc covers" and backfill
warmed one cover per track (~520k for ~170k tracks), filling album/ with mf-*
dirs instead of ~one cover per album.

Treat a release as having distinct disc covers only when each disc has a single
consistent cover that differs across discs (genuine box set); per-song ids now
collapse to one cover per album. Mirror the same fix in the TS
albumHasDistinctDiscCovers used by on-demand warming. Adds regression tests on
both sides.

* docs(changelog): record per-song cover over-fetch fix (PR #944)

Give the album/mf-* over-fetch fix its own [1.47.0] Fixed entry and a
settingsCredits line under PR #944.

* feat(cover): log failed cover downloads with album/artist name

A non-200 (or network-failed) getCoverArt download was swallowed silently. Now
the failure is logged with the resolved album/artist name and the server error,
so a server refusing covers under backfill load (5xx/429/timeouts) is visible.

- cover_resolve: describe_cover_entity() resolves a human label from the local
  index (album "Name" — Artist / artist "Name"), best-effort with id fallback.
- cover_cache: log_cover_fetch_failure() in ensure_inner logs on the download
  error path; threads optional library_server_id through CoverCacheEnsureArgs so
  the name lookup happens only on failure. Backfill logs at normal level,
  on-demand misses at debug level.

src/config/settingsCredits.ts

@@ -145,6 +145,7 @@ const CONTRIBUTOR_ENTRIES = [
       'Live Search: scoped browse on Artists, Albums, New Releases, Tracks, and Composers — header badge, ghost restore, album title FTS, session stash (PR #938)',
       'Performance: idle Rust CPU — backfill coordinator park, probe overlay stability, throttled idle polls, lazy cover prefetch restore (PR #939)',
       'Cover backfill: disk-free idle gate, snapshot-diff worklist, live-tunable parallelism, transient-error retries, and memoized offline & cache stats to stop idle CPU spin (PR #943)',
+      'Cover art: fix per-song cover over-fetch on Navidrome — only genuine per-disc artwork expands, collapsing ~520k per-track fetches to one cover per album (PR #944)',
     ],
   },
   {