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/migrations/20261019204122341-deletion-notices/definition_project.json

@@ -0,0 +1,29 @@
+{
+  "__className__": "serverpod.DatabaseDefinition",
+  "schemaVersion": 2,
+  "moduleName": "halkyon_todo",
+  "tables": [],
+  "installedModules": [
+    {
+      "__className__": "serverpod.DatabaseMigrationVersion",
+      "module": "halkyon_base",
+      "version": "20261019202835333-deletion-notices"
+    },
+    {
+      "__className__": "serverpod.DatabaseMigrationVersion",
+      "module": "serverpod",
+      "version": "20260824182259319"
+    },
+    {
+      "__className__": "serverpod.DatabaseMigrationVersion",
+      "module": "serverpod_auth_idp",
+      "version": "20260924105404509"
+    },
+    {
+      "__className__": "serverpod.DatabaseMigrationVersion",
+      "module": "serverpod_auth_core",
+      "version": "20260924105232991"
+    }
+  ],
+  "migrationApiVersion": 1
+}