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/password/password_change_test.dart

@@ -1,3 +1,4 @@
+import 'dart:async';
 import 'dart:typed_data';
 
 import 'package:clock/clock.dart';
@@ -7,6 +8,8 @@ import 'package:halkyon_base_server/src/password/password_endpoint.dart'
     show changesBegunPerSession;
 import 'package:halkyon_base_server/src/password/passwords.dart'
     show writeNewPassword;
+import 'package:halkyon_base_server/src/register/sessions_ended.dart'
+    show sessionNoticesDelivered;
 import 'package:serverpod/serverpod.dart';
 import 'package:serverpod_auth_idp_server/core.dart';
 import 'package:test/test.dart';
@@ -24,6 +27,10 @@ final class _Members implements MemberNotifier {
   final sessionsTold = <(Uri, UuidValue, DateTime, UuidValue?)>[];
   bool answering = true;
 
+  /// What a member's answer waits for, as one out of reach keeps it
+  /// waiting; none by default.
+  Future<void>? answerAfter;
+
   @override
   Future<bool> accountGone(Uri member, UuidValue account) async => answering;
 
@@ -34,6 +41,7 @@ final class _Members implements MemberNotifier {
     required DateTime endedBefore,
     UuidValue? keepSession,
   }) async {
+    await answerAfter;
     sessionsTold.add((member, account, endedBefore, keepSession));
     return answering;
   }
@@ -241,6 +249,7 @@ void main() {
           final sessions = await sessionsOf(anna);
           expect(sessions, contains(annasPhone));
           expect(sessions, isNot(contains(annasLaptop)));
+          await sessionNoticesDelivered();
           final (member, account, endedBefore, kept) =
               members.sessionsTold.first;
           expect(member, _todo);
@@ -307,6 +316,7 @@ void main() {
             kept,
           );
           expect(await sessionsOf(anna), {annasPhone, annasLaptop});
+          await sessionNoticesDelivered();
           expect(members.sessionsTold, isEmpty);
         });
 
@@ -566,10 +576,23 @@ void main() {
           );
         });
 
+        test('is done without waiting for the members: one out of reach '
+            "holds no change up (the full reading's L5)", () async {
+          final reached = Completer<void>();
+          members.answerAfter = reached.future;
+          await change();
+          expect(members.sessionsTold, isEmpty);
+          reached.complete();
+          await sessionNoticesDelivered();
+          expect(members.sessionsTold, hasLength(1));
+          expect(await SessionsEndedNotice.db.count(session), 0);
+        });
+
         test('tells a member that missed it again, hourly, until it '
             'confirms', () async {
           members.answering = false;
           await change();
+          await sessionNoticesDelivered();
           expect(await SessionsEndedNotice.db.count(session), 1);
 
           members.answering = true;
@@ -1042,6 +1065,7 @@ void main() {
           expect(sessions, hasLength(1));
           expect(sessions, isNot(contains(annasPhone)));
           expect(sessions, isNot(contains(annasLaptop)));
+          await sessionNoticesDelivered();
           expect(members.sessionsTold.single.$2, anna);
           expect(members.sessionsTold.single.$4, isNull);
         });

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,