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/urls.py

@@ -15,6 +15,7 @@ urlpatterns = [
     path("profile/photo/", views.profile_photo, name="profile_photo"),
     path("profile/email/verify/<str:token>/", views.verify_profile_email, name="verify_profile_email"),
     path("profile/email/confirm/", views.confirm_profile_email, name="confirm_profile_email"),
+    path("profile/newsletter/", views.newsletter_preference, name="newsletter"),
     path("checkout/<slug:tier>/", views.checkout, name="checkout"),
     path("billing/", views.portal, name="portal"),
     path("stripe/webhook/", views.webhook, name="webhook"),