fix(cover): follow connect-URL flips in library cover backfill (#952)

* fix(cover): follow connect-URL flips in library cover backfill

The native cover backfill was configured once with a snapshot of the runtime
connect URL. When a laptop moved off the LAN, the smart endpoint switch flipped
the sticky connect URL to the public address (playback/UI covers rebuild it per
request and followed it), but backfill kept fetching from the now-unreachable
local address — flooding the log with "error sending request" failures.

Make the connect cache observable (notify on effective flips) and have the
backfill hook reconfigure when the resolved URL changes, forcing a pass so the
.fetch-failed backoff from the stale address is cleared and those covers retry
on the reachable endpoint.

* docs(changelog): note cover backfill endpoint-switch fix (PR #952)

* fix(cover): abort stale backfill pass + rerun on connect-URL flip

Frontend reconfigure alone didn't fully cover the boot case: at startup the
first backfill pass starts on the primary (LAN) URL before the reachability
probe resolves, so when the probe flips to public the forced rerun was dropped
by the pass_running guard and the slow LAN pass (every cover timing out) ran to
completion with nothing re-running on the reachable address.

set_session now bumps a session generation; the running pass checks it on every
focus gate and abandons promptly when the URL flips (same server_index_key, new
rest_base_url). try_schedule_full_pass records a rerun when a pass is in flight
and drains it once the abandoned pass returns, so a fresh forced pass runs on
the new address.

* fix(cover-backfill): resolve connect URL per fetch instead of baking it into the queue

The backfill worklist no longer carries a URL. Each cover fetch reads the
current reachable address live from a single worker cell, so a LAN↔public flip
is honoured even by the pass already in flight — its remaining covers download
against the new endpoint without aborting/rebuilding the worklist.

- Drop rest_base_url from CoverBackfillSession; add live base_url cell read in
  ensure_one. Remove the session-generation abort machinery (no longer needed).
- New lightweight library_cover_backfill_set_base_url command pushes the URL on
  every connect-cache flip; a real change clears the stale .fetch-failed backoff
  and runs a forced pass so covers that timed out on the old address retry.
- Split useLibraryCoverBackfill into a configure effect (server/creds/strategy)
  and a flip effect that only pushes the URL.
- Keep a single rerun_pending flag for the boot case (flip mid-pass), since the
  finished pass re-arms the idle gate.

CHANGELOG.md

@@ -696,6 +696,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * It now treats a release as multi-disc only when each disc has a single consistent cover that differs across discs (a genuine box set); per-song ids collapse to one cover per album (≈ albums + artists). Fixed on both the Rust backfill path and the on-demand TS `albumHasDistinctDiscCovers`.
 * Failed cover downloads are now logged with the album/artist name and the server error (e.g. `fetch failed for album "X" — Artist (coverArtId=…): cover HTTP 503`). Backfill failures log at the normal level; incidental on-demand misses stay at the debug level.
 
+### Cover backfill — follow the local/public endpoint switch
+
+**By [@cucadmuh](https://github.com/cucadmuh), PR [#952](https://github.com/Psychotoxical/psysonic/pull/952)**
+
+* On a dual-address server, library cover backfill was configured once with a snapshot of the connect URL and never followed the smart LAN↔public switch. Starting already off the LAN — or moving off it mid-session — (internet up, playback already on the public address) left backfill hammering the now-unreachable local address and flooding the log with `error sending request` failures.
+* The backfill worklist no longer carries a URL: each cover fetch now reads the current reachable address live, so a LAN↔public flip is honoured even by the pass already in flight (its remaining covers download against the new endpoint). The connect cache is observable and pushes the resolved URL to the native worker on every flip; a real change clears the stale `.fetch-failed` backoff and runs a forced pass so the handful of covers attempted against the old address retry on the reachable one. This also covers the boot case where the initial pass starts on the primary URL before the first reachability probe resolves. On-demand UI / playback covers already followed the switch.
+
 ## [1.46.0] - 2026-05-18
 
 > **🙏 Special thanks to [@zz5zz](https://github.com/zz5zz)** for his tireless quirk-spotting and bug reports on the [Psysonic Discord](https://discord.gg/AMnDRErm4u) — several of the polish fixes in this release landed directly off the back of his messages.