Email queue + async sending; legal settings and placeholders

- Add in-memory email queue with rate limiting (MAX_EMAILS_PER_HOUR)
- Bulk send to event attendees now queues and returns immediately
- Frontend shows 'Emails are being sent in the background'
- Legal pages, settings, and placeholders updates

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/.env.example

@@ -67,3 +67,8 @@ SMTP_TLS_REJECT_UNAUTHORIZED=true
 # SendGrid: SMTP_HOST=smtp.sendgrid.net, SMTP_PORT=587, SMTP_USER=apikey, SMTP_PASS=your_api_key
 # Mailgun: SMTP_HOST=smtp.mailgun.org, SMTP_PORT=587
 # Amazon SES: SMTP_HOST=email-smtp.us-east-1.amazonaws.com, SMTP_PORT=587
+
+# Email Queue Rate Limiting
+# Maximum number of emails that can be sent per hour (default: 30)
+# If the limit is reached, queued emails will pause and resume automatically
+MAX_EMAILS_PER_HOUR=30