Membership: newsletter opt-in at signup and on the profile

An unticked checkbox on registration subscribes the verified address to
the listmonk list (preconfirmed) once the account exists. The profile
page gets a subscribe/unsubscribe toggle, and verified email changes
follow through to listmonk. Newsletter outages never block accounts.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

membership/members/models.py

@@ -25,6 +25,7 @@ class Registration(models.Model):
     token_hash = models.CharField(max_length=64, primary_key=True)
     uid = models.CharField(max_length=48)
     email = models.EmailField()
+    newsletter = models.BooleanField(default=False)
     expires_at = models.DateTimeField(db_index=True)