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/forms.py

@@ -1,10 +1,21 @@
 from django import forms
+from django.conf import settings
 from django.contrib.auth.password_validation import validate_password
 
 
+NEWSLETTER_LABEL = "Email me when new works premiere (about twice a month). Unsubscribe any time."
+
+
 class RegistrationForm(forms.Form):
     uid = forms.RegexField(regex=r"^[a-z][a-z0-9._-]{2,47}$", max_length=48, label="Username", help_text="3–48 lowercase letters, numbers, dots, underscores or hyphens. Start with a letter.")
     email = forms.EmailField(max_length=254)
+    # Explicit opt-in: unticked by default, and only offered when configured.
+    newsletter = forms.BooleanField(required=False, label=NEWSLETTER_LABEL)
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        if not settings.LISTMONK_ENABLED:
+            del self.fields["newsletter"]
 
     def clean_email(self):
         return self.cleaned_data["email"].strip().casefold()