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/migrations/0006_registration_newsletter.py

@@ -0,0 +1,8 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [("members", "0005_profile_email_change")]
+    operations = [
+        migrations.AddField(model_name="registration", name="newsletter", field=models.BooleanField(default=False)),
+    ]