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/mail/email_changed_notice/body.txt

@@ -0,0 +1,11 @@
+{% load i18n tz %}{% autoescape off %}{% blocktrans with name=name %}Hello {{ name }},{% endblocktrans %}
+
+{% timezone zone_name %}{% blocktrans with when=changed_at|date:"j. F Y, H:i" zone=changed_at|date:"T" address=new_address %}the e-mail address of your portal account was changed on {{ when }} ({{ zone }}) to {{ address }}.{% endblocktrans %}{% endtimezone %}
+
+{% trans "If you did this, you do not need to do anything. If you did not, open this link to restore this address. The link is valid for 24 hours:" %}
+
+{{ link }}
+
+{% trans "Undoing signs you out on all devices and informs the administrator." %}
+
+{% trans "This e-mail never contains your password." %}{% endautoescape %}