feat(web): subscribe to SSE and refresh home/playlists on playlist.system_rebuilt

The web client only ever SENT events; it had no inbound SSE listener, so a
tab left open across the daily system-playlist rebuild kept showing
yesterday's home + playlist snapshots until a manual reload (the stale-
browse-view bug behind #968). Add useServerEvents(): opens /api/events/stream
while authenticated and, on playlist.system_rebuilt, invalidates the home,
playlists, and system-playlist-status query caches. Deliberately does not
disturb the active playback queue — that self-heals on the failure path.
Issue #968.

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

web/src/lib/serverEvents.svelte.ts

@@ -0,0 +1,37 @@
+import { queryClient } from '$lib/query/client';
+import { qk } from '$lib/api/queries';
+import { user } from '$lib/auth/store.svelte';
+
+// Inbound server events over SSE (#968). The web client's OUTBOUND half
+// (play events) lives in player/events.svelte.ts; this is the inbound
+// listener it never had. Today it reacts to `playlist.system_rebuilt` — the
+// daily 03:00 rebuild or a manual refresh — by invalidating the home +
+// system-playlist query caches so a tab left open across the rebuild stops
+// serving yesterday's snapshot (the stale-browse-view bug). Active playback
+// self-heals separately on the failure path (store.handleLoadFailure); we
+// deliberately do NOT yank a playing queue here — that would interrupt a
+// mid-song listen to restart the new mix at track 0.
+
+function onSystemRebuilt(): void {
+  queryClient.invalidateQueries({ queryKey: qk.home() });
+  // Prefix match invalidates every kind ('user' | 'system' | 'all').
+  queryClient.invalidateQueries({ queryKey: ['playlists'] });
+  queryClient.invalidateQueries({ queryKey: qk.systemPlaylistsStatus() });
+}
+
+// Wire once from +layout.svelte's mount. Opens the stream only while
+// authenticated and closes it on logout; EventSource auto-reconnects on
+// transient network drops.
+export function useServerEvents(): void {
+  $effect(() => {
+    if (!user.value) return;
+    // Absent under SSR / jsdom — the listener simply no-ops there.
+    if (typeof EventSource === 'undefined') return;
+    const es = new EventSource('/api/events/stream');
+    es.addEventListener('playlist.system_rebuilt', onSystemRebuilt);
+    return () => {
+      es.removeEventListener('playlist.system_rebuilt', onSystemRebuilt);
+      es.close();
+    };
+  });
+}

web/src/routes/+layout.svelte

@@ -23,6 +23,7 @@
   } from '$lib/player/store.svelte';
   import { useMediaSession } from '$lib/player/mediaSession.svelte';
   import { useEventsDispatcher } from '$lib/player/events.svelte';
+  import { useServerEvents } from '$lib/serverEvents.svelte';
   import { useGlobalShortcuts } from '$lib/player/shortcuts.svelte';
   import { applyMetaThemeColor } from '$lib/theme/applyMetaThemeColor.svelte';
   import { audioLoader } from '$lib/player/audioLoader';
@@ -132,6 +133,7 @@
 
   useMediaSession();
   useEventsDispatcher();
+  useServerEvents();
   useGlobalShortcuts();
   applyMetaThemeColor();
 </script>