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_client/lib/migrations/migration_registry.dart

@@ -17,6 +17,7 @@ part '20261009224606274-feedback/migration.dart';
 part '20261012162411835-studyset-deleted-at/migration.dart';
 part '20261019121406578-halkyon-base/migration.dart';
 part '20261019201023295-profiles-in-halkyon-base/migration.dart';
+part '20261019204112282-deletion-notices/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -36,6 +37,7 @@ class MigrationRegistry {
     _Migration20261012162411835_studyset_deleted_at(),
     _Migration20261019121406578_halkyon_base(),
     _Migration20261019201023295_profiles_in_halkyon_base(),
+    _Migration20261019204112282_deletion_notices(),
   ];
 
   /// List of all client-side database migration versions.