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-tauri/src/cover_cache/backfill_worker.rs

@@ -271,6 +271,7 @@ async fn ensure_one(
         username: session.username,
         password: session.password,
         library_bulk: true,
+        library_server_id: Some(session.library_server_id),
     };
     let _ = CoverCacheState::ensure_inner(&st, &app, &args, Some(http_sem)).await;
 }