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/forms.py

@@ -5,6 +5,8 @@ from django import forms
 from django.utils.translation import gettext
 from django.utils.translation import gettext_lazy as _
 
+from portal.mailer.address import is_valid_address
+
 MAX_PASSWORD_LENGTH = 256  # Active Directory's limit
 
 
@@ -163,3 +165,27 @@ class DetailsForm(forms.Form):
     def apply_errors(self, errors: dict[str, str]) -> None:
         for name, message in errors.items():
             self.add_error(name, message)
+
+
+# -- e-mail change ----------------------------------------------------------------------------
+
+MAX_ADDRESS_LENGTH = 254
+
+
+class EmailChangeForm(forms.Form):
+    new_email = forms.CharField(
+        strip=True,
+        required=False,  # the messages below say it better than the default
+        widget=forms.EmailInput(),
+    )
+    current_password = _password_field()
+
+    def clean_new_email(self):
+        value = self.cleaned_data["new_email"]
+        if not value:
+            raise forms.ValidationError(_("Please fill in this field."))
+        if len(value) > MAX_ADDRESS_LENGTH:
+            raise forms.ValidationError(_("This e-mail address is too long."))
+        if not is_valid_address(value):
+            raise forms.ValidationError(_("Please enter a valid e-mail address."))
+        return value