feat(accounts): one account across an instance's services (ADR 0016)

halkyon_base runs as the instance's register or as one of its members.
The register keeps the ways in, handles and profiles; members ask it
over HTTP with the instance's shared secret (instanceSecret), and open
sessions of their own for its accounts, under the same account ID.

- AccountsMode from the environment: INSTANCE_MEMBERS on the register,
  ACCOUNT_REGISTER_URL on a member. Learn always holds the register
  (Explore joins the profiles); Todo is a member where it is set.
- AccountRegister: LocalRegister, RemoteRegister, an in-memory one in
  tests. Calls between services on the web server, refused without the
  secret (constant-time compare); a server needing it doesn't start
  without it.
- Deleting an account deletes it everywhere: notices per member kept in
  the deletion's transaction (account_deletion_notice), told at once and
  hourly until confirmed; members prune daily whom the register no longer
  knows, never while it knows none of them or can't be reached.
- Deploy: INSTANCE_SECRET and INSTANCE_MEMBERS in compose and .env.

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

apps/learn/halkyon_learn_server/test/integration/test_tools/serverpod_test_tools.dart

@@ -2047,6 +2047,21 @@ class _PruningFutureCall {
       await _localUniqueSession.close();
     }
   }
+
+  Future<void> tellMembersOfDeletions(
+    _ist.TestSessionBuilder sessionBuilder,
+  ) async {
+    var _localUniqueSession =
+        (sessionBuilder as _ist.InternalTestSessionBuilder).internalBuild();
+    try {
+      await _iip9210p.PruningTellMembersOfDeletionsFutureCall().invoke(
+        _localUniqueSession,
+        null,
+      );
+    } finally {
+      await _localUniqueSession.close();
+    }
+  }
 }
 
 class _DictionaryReloadFutureCall {

apps/learn/halkyon_learn_server/test/unit/auth/learn_base_test.dart

@@ -1,3 +1,4 @@
+import 'package:halkyon_base_server/base.dart';
 import 'package:halkyon_learn_server/src/auth/learn_base.dart';
 import 'package:test/test.dart';
 
@@ -6,4 +7,36 @@ void main() {
       'apps accept', () {
     expect(learnBase().app, 'learn');
   });
+
+  group("Learn's part in the instance's accounts (ADR 0016)", () {
+    test('is the register, telling the members the operator names', () {
+      final accounts = learnAccounts(const {
+        'INSTANCE_MEMBERS': 'http://todo:8082',
+      });
+
+      expect(accounts.members, [Uri.parse('http://todo:8082')]);
+      expect(learnBase(accounts: accounts).accounts, same(accounts));
+    });
+
+    test('is the register with no members unless the operator names some', () {
+      expect(learnAccounts(const {}).members, isEmpty);
+      expect(learnBase().accounts, isA<HoldsRegister>());
+    });
+
+    test('is never a member: Explore searches the profiles in its own '
+        'database', () {
+      expect(
+        () => learnAccounts(const {
+          'ACCOUNT_REGISTER_URL': 'http://todo:8082',
+        }),
+        throwsA(
+          isA<StateError>().having(
+            (error) => error.message,
+            'message',
+            contains('ACCOUNT_REGISTER_URL'),
+          ),
+        ),
+      );
+    });
+  });
 }