feat: add OIDC sign-in, roles, sessions and client address rule

Authorization-code flow with PKCE through an idp adapter (with a fake), ID
token validation, identity and roles resolved from the directory with a short
cache, refusal of unknown, disabled and locked accounts with one generic page,
sessions that end when the account becomes unavailable, sign-out that ends the
server-side session, role decorators and the trusted-proxy client address rule.
Adds the OIDC_* and PUBLIC_BASE_URL configuration.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N3t4WL7bd9bdHHNjEsxzqC

portal/accounts/urls.py

@@ -0,0 +1,11 @@
+from django.urls import path
+
+from . import views
+
+urlpatterns = [
+    path("", views.home, name="home"),
+    path("auth/login", views.login, name="login"),
+    path("auth/callback", views.callback, name="callback"),
+    path("auth/logout", views.logout, name="logout"),
+    path("auth/signed-out", views.signed_out, name="signed-out"),
+]