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/todo/halkyon_todo_server/lib/server.dart

@@ -9,8 +9,12 @@ Future<void> run(List<String> args) async {
   // The generated Serverpod is already connected with the generated code.
   final pod = Serverpod(args);
   // What every Halkyon server has (ADR 0014): sign-in, the account and its
-  // deletion, the pruning, the instance's basics, set up for Todo.
-  HalkyonBase.configure(pod, todoBase());
+  // deletion, the pruning, the instance's basics, set up for Todo; a member
+  // of the instance's register where the operator names one (ADR 0016).
+  HalkyonBase.configure(
+    pod,
+    todoBase(accounts: AccountsMode.fromEnvironment()),
+  );
   // Token managers validate and issue authentication keys; identity
   // providers are the ways people can sign in (halkyon_base's
   // auth_config.dart).
@@ -19,6 +23,10 @@ Future<void> run(List<String> args) async {
     identityProviderBuilders: identityProviderBuilders(pod),
   );
 
+  // The calls between the instance's services, on the web server
+  // (ADR 0016); Caddy never forwards them.
+  addInstanceRoutes(pod);
+
   await pod.start();
 
   await schedulePruning(pod);