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/unavailable.html

@@ -0,0 +1,7 @@
+{% extends "accounts/base.html" %}
+{% load i18n %}
+{% block title %}{% trans "Not available" %}{% endblock %}
+{% block content %}
+<h1>{% trans "Not available" %}</h1>
+<p>{% trans "This is not possible right now. Please try again later." %}</p>
+{% endblock %}