Restructure step 1: lasting identity, presence and protocol 16

Server and core: version ranges on both sides with a coded refusal; a
client id maps to the same member and token for the server's life; a
drop is `gone` for a 10-minute grace keeping ready, presence, queue
items, relay and avatar; `bye` leaves at once; ordered takeover (seq,
replaced/stale); presence joining/in/away/gone with one PresencePolicy;
joiners never pause playback and hold a held gate at most ~15 s;
state.hold; feed ids, seq and replay; the relay lives through its
source's grace and continues on re-dial; queue `by` lasting, auto
items and cancel; MediaRef carried; client ids never logged.

App: persisted client id and seq; Connection with unstable/lost/
replaced states, backoff with jitter, no lost/back pair per flap; a drop
keeps the session (state, player, media, relay source); presence in the
strip and members card; sit out = away intent; feed matched by id;
auto sit-out (idle when not playing, minimised for a while; never when
alone) replaces auto-unready; bye on close and Leave, not on update
restart; relay source re-dials through its grace.

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

packages/server/test/jellyfin_relay_live_test.dart

@@ -54,8 +54,7 @@ void main() {
       ws.add(jsonEncode(HelloMessage(
               proto: RelayConstants.protoVersion,
               secret: '',
-              user: const HelloUser(name: 'live'),
-              media: const HelloMedia(name: '', size: 0, duration: 0))
+              user: const HelloUser(name: 'live'))
           .toJson()));
       final welcome = await frames.firstWhere((f) => f['t'] == 'welcome').timeout(const Duration(seconds: 5));
       ws.add(jsonEncode(SwapIntent(