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>

packages/halkyon_base_server/test/integration/support/fixtures.dart

@@ -26,6 +26,7 @@ void useBase(
   TestSessionBuilder sessionBuilder, {
   List<PersonDataDeletion> deletePersonData = const [],
   ProfilePageNote? profilePageNote,
+  AccountsMode accounts = const HoldsRegister(),
 }) {
   final pod = sessionBuilder.build().serverpod;
   HalkyonBase.configure(
@@ -37,6 +38,7 @@ void useBase(
       minimumAppBuild: testMinimumAppBuild,
       deletePersonData: deletePersonData,
       profilePageNote: profilePageNote,
+      accounts: accounts,
     ),
   );
   AuthServices.set(