feat(web/player): self-heal a stale system-playlist / radio queue on total failure

When the whole queue proves unplayable (e.g. a tab left open across the
daily system-playlist rebuild — the exact stale-snapshot case), the player
now re-pulls the fresh snapshot and resumes instead of dead-ending on
"Try again". The seeder hands the store an opaque refetch closure so the
store stays decoupled from the playlist API and the per-artist
(songs_like_artist) identity problem: single-instance variants re-pull via
systemShuffle, per-artist mixes via getPlaylist(id), radio re-seeds from
its track. Bounded to one self-heal per exhaustion (reset on the next
successful play) so a still-broken refresh can't loop; "Try again" stays
the genuine last resort. Wired from PlaylistCard, the playlist detail page,
and playRadio. Issue #968.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/player/store.svelte.ts

@@ -89,6 +89,17 @@ let _queueSource = $state<string | null>(null);
 // looping forever. Reset on a successful play and on a fresh playQueue.
 let _failureStreak = 0;
 
+// Track B (#968): when a queue is seeded from a refreshable source (a system
+// playlist or radio), the seeder hands us a closure that re-pulls the fresh
+// snapshot. On total failure (whole queue unplayable — likely a stale tab
+// left open across the daily rebuild) we call it once to self-heal before
+// surfacing the dead-end. Kept opaque so the store stays decoupled from the
+// playlist API and the per-artist (songs_like_artist) identity problem.
+let _queueRefetch: (() => Promise<TrackRef[]>) | null = null;
+let _selfHealInFlight = false;
+let _selfHealAttempts = 0;
+const MAX_SELF_HEAL_ATTEMPTS = 1;
+
 let _audioEl: HTMLAudioElement | null = null;
 
 export const player = {
@@ -115,13 +126,20 @@ export function registerAudioEl(el: HTMLAudioElement | null): void {
 export function playQueue(
   tracks: TrackRef[],
   startIndex = 0,
-  opts: { shuffle?: boolean; source?: string | null } = {},
+  opts: {
+    shuffle?: boolean;
+    source?: string | null;
+    refetch?: () => Promise<TrackRef[]>;
+  } = {},
 ): void {
   _radioSeedId = null;  // M4c: non-radio enqueue clears the radio refresh state
   // #415: a fresh queue resets the system-playlist source. Set only
   // when seeded from a system playlist (server already returned the
   // rotation-aware order, so no client shuffle in that path).
   _queueSource = opts.source ?? null;
+  // #968: a fresh play replaces the self-heal closure + resets its budget.
+  _queueRefetch = opts.refetch ?? null;
+  _selfHealAttempts = 0;
   if (opts.shuffle && tracks.length > 1) {
     // Fisher-Yates over the whole list. startIndex is ignored — the
     // caller is asking for "random play from this pool," so the first
@@ -271,6 +289,7 @@ export function reportStateFromAudio(
       _state = 'playing';
       _error = null;
       _failureStreak = 0;
+      _selfHealAttempts = 0;
       return;
     case 'paused':
       _state = 'paused';
@@ -323,10 +342,53 @@ function handleLoadFailure(detail?: string): void {
     _error = null;
     return;
   }
+  // Whole queue is unplayable. If it came from a refreshable source, the
+  // snapshot is probably stale — re-pull it and resume before giving up.
+  if (trySelfHeal()) return;
   _state = 'error';
   _error = detail ?? 'Playback failed.';
 }
 
+// #968: re-pull a stale refreshable queue. Returns true if a self-heal was
+// started (caller must not dead-end). Bounded to one attempt per exhaustion
+// (reset on the next successful play) so a still-broken refresh can't loop.
+function trySelfHeal(): boolean {
+  if (_selfHealInFlight) return true;
+  if (_queueRefetch === null) return false;
+  if (_selfHealAttempts >= MAX_SELF_HEAL_ATTEMPTS) return false;
+  _selfHealAttempts++;
+  _selfHealInFlight = true;
+  _state = 'loading';
+  _error = null;
+  const refetch = _queueRefetch;
+  void refetch()
+    .then((refs) => {
+      if (refs.length > 0) {
+        // Re-seed in place — keep _queueSource / _queueRefetch so the new
+        // snapshot can itself self-heal, and don't reset _selfHealAttempts
+        // (only a successful 'playing' clears the budget).
+        _queue = refs;
+        _index = 0;
+        _position = 0;
+        _duration = 0;
+        _failureStreak = 0;
+        _state = 'loading';
+        _error = null;
+      } else {
+        _state = 'error';
+        _error = 'Nothing playable in this mix right now.';
+      }
+    })
+    .catch(() => {
+      _state = 'error';
+      _error = 'Couldn’t refresh this mix. Try again.';
+    })
+    .finally(() => {
+      _selfHealInFlight = false;
+    });
+  return true;
+}
+
 export function enqueueTrack(t: TrackRef): void {
   _radioSeedId = null;  // M4c
   _queue = [..._queue, t];
@@ -371,12 +433,19 @@ export function playNextMany(ts: TrackRef[]): void {
   _queue = [..._queue.slice(0, next), ...ts, ..._queue.slice(next)];
 }
 
-export async function playRadio(seedTrackId: string): Promise<void> {
+async function fetchRadioTracks(seedTrackId: string): Promise<TrackRef[]> {
   const resp = await api.get<RadioResponse>(
     `/api/radio?seed_track=${encodeURIComponent(seedTrackId)}`
   );
-  if (resp.tracks.length === 0) return;
-  playQueue(resp.tracks, 0);
+  return resp.tracks;
+}
+
+export async function playRadio(seedTrackId: string): Promise<void> {
+  const tracks = await fetchRadioTracks(seedTrackId);
+  if (tracks.length === 0) return;
+  // #968: hand the player a self-heal closure so a fully-stale radio queue
+  // re-seeds from the same track instead of dead-ending.
+  playQueue(tracks, 0, { refetch: () => fetchRadioTracks(seedTrackId) });
   _radioSeedId = seedTrackId;  // M4c: set AFTER playQueue (which clears it)
 }