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 {