feat: add e-mail change with confirmation and undo

Page at /email asks for the new address and the current password (shared failure counter with the password page, five requests per hour) and mails a one-hour confirmation link to the new address. Confirming writes the address and mails the old address a 24-hour undo link; undoing restores it, ends every session of the user and notifies the Admin (ADMIN_NOTIFY_EMAIL). Link pages show on GET and apply on POST; tokens are stored only as hashes, single use, and are blanked from every log line. New table owned by profile, German text, tests; review findings logged in the plan.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FocPJ75XPma7TcuYEMy8zm

portal/templates/profile/details.html

@@ -29,7 +29,7 @@
   <dt>{% trans "User name" %}</dt>
   <dd>{{ details.username }}</dd>
   <dt>{% trans "E-mail address" %}</dt>
-  <dd>{% if details.mail %}{{ details.mail }}{% else %}&ndash;{% endif %}</dd>
+  <dd>{% if details.mail %}{{ details.mail }}{% else %}&ndash;{% endif %} <a href="{% url 'email' %}">{% trans "Change e-mail address" %}</a></dd>
 </dl>
 {% endif %}
 <form method="post" action="{% url 'profile' %}" class="card" data-busy-on-submit novalidate>