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/test/session_test.dart

@@ -80,8 +80,14 @@ void main() {
   });
 
   group('core welcome + proto', () {
-    test('proto 7: Jellyfin refs and relay', () {
-      expect(RelayConstants.protoVersion, 7);
+    test('proto 8: relayed media', () {
+      expect(RelayConstants.protoVersion, 8);
+      const m = HelloMedia(name: 'a', size: 1, duration: 0, relay: MediaRelay(source: 'm1', live: true));
+      final back = HelloMedia.fromJson(Map<String, Object?>.from(m.toJson()));
+      expect(back.relay!.source, 'm1');
+      expect(back.relay!.live, isTrue);
+      expect(back.relay!.fetch, isFalse);
+      expect(const MediaRelay().toJson(), isEmpty);
     });
 
     test('welcome round-trips with token + member id', () {