Protocol 14: a warm relay preloads relayed Up next items for everyone

The server keeps one small relay (32 MiB ahead, 8 behind) for Up next's
first relayed item, keyed by its queue id at a URL that stays the same
after the swap: fetched by the server for server-fetch links, fed by
the adder's app for through-me links and files, by a member with an
account for member-passed Jellyfin items. Every app preloads from it;
the swap promotes it with its buffer. Dropped when the item stops
being first or its adder leaves. Measured switch: ~1.6 s cold, 20-40 ms
preloaded. ROADMAP gains the research item on server-side fetching of
member-resolved URLs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/server/lib/src/sync_server.dart

@@ -94,7 +94,7 @@ class SyncServer {
       );
     });
     mountSessionMediaRoute(_app, sessions);
-    mountRelayRoutes(_app, sessions, () => hub.relay);
+    mountRelayRoutes(_app, sessions, relayAt: hub.relayAt, sourceOf: hub.sourceOf);
     mountAvatarRoute(_app, hub);
     mountArtRoute(_app, hub);
     mountJellyfinRelayRoute(
@@ -108,8 +108,8 @@ class SyncServer {
       return 'http://${authority(dialledHost)}/m/$sid/$token/$name';
     }, relayUrlBuilder: (sid, token, itemId, dialledHost) =>
         'http://${authority(dialledHost)}/j/$sid/$token/${Uri.encodeComponent(itemId)}',
-        relayMediaUrlBuilder: (sid, token, mediaId, name, tracks, dialledHost) =>
-            relayViewerUrl('http://${authority(dialledHost)}/r/$sid/$token/$mediaId', name, tracks: tracks));
+        relayMediaUrlBuilder: (sid, token, key, name, tracks, dialledHost) =>
+            relayViewerUrl('http://${authority(dialledHost)}/r/$sid/$token/$key', name, tracks: tracks));
     _app.fallback = (final req) => Response.notFound(
           body: Body.fromString('not found'),
         );
@@ -181,7 +181,7 @@ class SyncServer {
     _heartbeat = null;
     _relayClient?.close(force: true);
     _relayClient = null;
-    hub.setRelay(null);
+    hub.dropRelays();
     await hub.closeAllSockets();
     await _app.close();
     _server = null;