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/profile/urls.py

@@ -6,4 +6,7 @@ urlpatterns = [
     path("profile", views.profile, name="profile"),
     path("password", views.password, name="password"),
     path("password/done", views.password_done, name="password-done"),
+    path("email", views.email, name="email"),
+    path("email/confirm/<str:token>", views.email_confirm, name="email-confirm"),
+    path("email/undo/<str:token>", views.email_undo, name="email-undo"),
 ]