refactor(auth)!: sign-in and the account come from halkyon_base

Sign-in (anonymous, GitHub, email codes), an account's ways in and its
deletion, the pruning and the instance's basics move into the
halkyon_base module (ADR 0014), which every Halkyon server includes.
Learn configures it in server.dart and deletes its own data with a
person through the module's hook, in the order it did. The identity
tables move unchanged: the migration records versions only. The app
calls the module's endpoints; instance.info() stays as it was.

BREAKING CHANGE: the sign-in endpoints moved into the module, so apps up
to build 20 can no longer sign in or start; minimumAppBuild is 21.
Deploy this only together with the release of build 21.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_server/config/generator.yaml

@@ -8,6 +8,9 @@ server_test_tools_path: test/integration/test_tools
 modules:
   serverpod_offline_sync:
     nickname: offline_sync
+  # What every Halkyon server has (ADR 0014): `client.modules.base` in the app.
+  halkyon_base:
+    nickname: base
 
 experimental_features:
   databaseSync: true