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/pubspec.yaml

@@ -11,6 +11,16 @@ resolution: workspace
 dependencies:
   clock: ^1.1.3
   crypto: ^3.0.7
+  # What every Halkyon server has (ADR 0014): signing in, the account and its
+  # deletion, pruning, the instance's basics. A Serverpod module in
+  # halkyon_shared, pinned to a signed commit of its main; for local work on
+  # both, a git-ignored pubspec_overrides.yaml at the workspace root points it
+  # at a checkout.
+  halkyon_base_server:
+    git:
+      url: https://code.halkyon.app/project-halkyon/halkyon_shared.git
+      path: packages/halkyon_base_server
+      ref: af1de799f402ef76ad4513052c3e0008155a92fb
   # Reads the pages profile links point to, for their rel="me" back-links.
   html: ^0.15.6
   # Mails email sign-in's codes over SMTP (ADR 0010).