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/config/urls.py

@@ -1,7 +1,8 @@
-from django.urls import path
+from django.urls import include, path
 
 from .health import healthz
 
 urlpatterns = [
     path("healthz", healthz, name="healthz"),
+    path("", include("portal.accounts.urls")),
 ]