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/templates/accounts/refused.html

@@ -0,0 +1,8 @@
+{% extends "accounts/base.html" %}
+{% load i18n %}
+{% block title %}{% trans "Sign-in not possible" %}{% endblock %}
+{% block content %}
+<h1>{% trans "Sign-in not possible" %}</h1>
+<p>{% trans "You cannot sign in to the portal with this account. If you think this is a mistake, contact your administrator." %}</p>
+<p><a href="{% url 'login' %}">{% trans "Try again" %}</a></p>
+{% endblock %}