Relay: a member's media through the server's buffer (proto 8)

Viewers who cannot reach the media get it through the server: a file on
any member's computer, a link passed on by a member ("Through me") or
fetched by the server ("Server fetches it", password-protected servers
only), and Jellyfin items for guests without an account, sourced by
whoever put them up with their own sign-in (or a pasted link's key),
which never leaves their machine. Fixes guests on a dedicated server
without a Jellyfin login being blocked.

The source dials /r/.../source (works behind NAT) and answers pulls with
an adaptive in-flight window (2 x rate x min RTT, 256 KiB..8 MiB). The
server keeps a memory-only buffer of 1 MiB slices sized like a viewer's
mpv buffer (pinned head/tail, 64 MiB back, 256 MiB ahead), fills it with
one sequential pull at a time (blocked viewers first, newest request
only), fetches nothing while nobody watches through it, and drops it on
the next swap or stop. Measured with mpv through shaped links
(tool/relay_e2e.dart): 400 Mbit/s seeks ~0.03 s; 40 Mbit/s no stalls,
3 viewers cost the source ~1x the bytes.

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

packages/server/lib/src/sync_server.dart

@@ -8,6 +8,7 @@ import 'package:sync_relay_core/core.dart';
 
 import 'jellyfin_relay.dart';
 import 'media_endpoint.dart';
+import 'relay_routes.dart';
 import 'server_hub.dart';
 import 'session.dart';
 
@@ -62,6 +63,7 @@ class SyncServer {
       );
     });
     mountSessionMediaRoute(_app, sessions);
+    mountRelayRoutes(_app, sessions, () => hub.relay);
     mountJellyfinRelayRoute(
         _app, sessions, () => hub.jellyfin, () => _relayClient ??= HttpClient());
     // advertiseHost is only an explicit override; otherwise echo what the client dialled.
@@ -72,7 +74,9 @@ class SyncServer {
       final name = Uri.encodeComponent(media?.name ?? 'file');
       return 'http://${authority(dialledHost)}/m/$sid/$token/$name';
     }, relayUrlBuilder: (sid, token, itemId, dialledHost) =>
-        'http://${authority(dialledHost)}/j/$sid/$token/${Uri.encodeComponent(itemId)}');
+        'http://${authority(dialledHost)}/j/$sid/$token/${Uri.encodeComponent(itemId)}',
+        relayMediaUrlBuilder: (sid, token, mediaId, name, dialledHost) =>
+            'http://${authority(dialledHost)}/r/$sid/$token/$mediaId/${Uri.encodeComponent(name)}');
     _app.fallback = (final req) => Response.notFound(
           body: Body.fromString('not found'),
         );
@@ -143,6 +147,7 @@ class SyncServer {
     _heartbeat = null;
     _relayClient?.close(force: true);
     _relayClient = null;
+    hub.setRelay(null);
     await hub.closeAllSockets();
     await _app.close();
     _server = null;