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/models.py

@@ -0,0 +1,25 @@
+from django.db import models
+
+
+class PendingEmailChange(models.Model):
+    """A one-time link for an e-mail change. Owned by `profile` (AD-19).
+
+    The token itself is never stored, only its SHA-256 hash (AD-9). A `confirm` row holds the
+    address the user asked for (`new_address`) and the one they had then (`old_address`). An
+    `undo` row holds the address to restore (`old_address`) and the one that was confirmed
+    (`new_address`).
+    """
+
+    class Kind(models.TextChoices):
+        CONFIRM = "confirm"
+        UNDO = "undo"
+
+    user_key = models.CharField(max_length=256, db_index=True)  # lower case
+    sam = models.CharField(max_length=256)  # exact sAMAccountName, for directory calls
+    kind = models.CharField(max_length=10, choices=Kind.choices)
+    token_hash = models.CharField(max_length=64, unique=True)
+    old_address = models.CharField(max_length=254, blank=True)
+    new_address = models.CharField(max_length=254)
+    created_at = models.DateTimeField()
+    expires_at = models.DateTimeField()
+    used_at = models.DateTimeField(null=True, blank=True)