feat: add password change

Password page with the AD policy listed first, ordered service (throttle limit below the lockout threshold, verification, local hints, directory change, ended sessions, audit and confirmation mail in one transaction), plain-language errors, German text and tests.

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

portal/templates/profile/password_done.html

@@ -0,0 +1,12 @@
+{% extends "base.html" %}
+{% load i18n %}
+{% block title %}{% trans "Password changed" %}{% endblock %}
+{% block content %}
+<h1>{% trans "Password changed" %}</h1>
+<div class="notice success" role="status">
+  <p>{% trans "Your password was changed." %}</p>
+</div>
+<p>{% trans "You were signed out on your other devices." %}</p>
+{% if mail %}<p>{% trans "A confirmation was sent to your e-mail address." %}</p>{% endif %}
+<p><a class="button" href="{% url 'home' %}">{% trans "Back to the start page" %}</a></p>
+{% endblock %}