fix(auth): members learn of ended sessions in the background

The full reading's L5: every change or reset of a password told the
members inside the request, up to 500 notices one after another with a
timeout of 10 seconds each, so a member out of reach held the change up
for minutes. Now announceSessionsEnded tells the members of this account's
notices alone, in the background on a session of its own; those not
confirmed wait for the hourly run as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/test/integration/register/register_test.dart

@@ -7,6 +7,8 @@ import 'package:halkyon_base_server/src/register/deletion_notices.dart';
 import 'package:halkyon_base_server/src/register/instance_api.dart';
 import 'package:halkyon_base_server/src/register/register_protocol.dart';
 import 'package:halkyon_base_server/src/register/remote_register.dart';
+import 'package:halkyon_base_server/src/register/sessions_ended.dart'
+    show sessionNoticesDelivered;
 import 'package:http/http.dart' as http;
 import 'package:http/testing.dart';
 import 'package:serverpod/serverpod.dart';
@@ -677,6 +679,7 @@ void main() {
               (account: annaOnTodo, endedBefore: now),
             );
             // Every member is told to end the account's sessions too.
+            await sessionNoticesDelivered();
             expect(members.sessionsTold.single, (_todo, annaOnTodo, now, null));
 
             final forReset = await remote.beginPasswordSignIn(
@@ -809,6 +812,7 @@ void main() {
                 password: device.newPassword(),
                 keepSession: annasPhone,
               );
+              await sessionNoticesDelivered();
               expect(members.sessionsTold.single, (
                 _todo,
                 annaOnTodo,