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/base.html

@@ -0,0 +1,13 @@
+{% load i18n %}<!doctype html>
+<html lang="{{ LANGUAGE_CODE }}">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<title>{% block title %}{% trans "Self-service portal" %}{% endblock %}</title>
+</head>
+<body>
+<main>
+{% block content %}{% endblock %}
+</main>
+</body>
+</html>