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>

frontend/src/app/admin/events/[id]/page.tsx

@@ -423,7 +423,6 @@ export default function AdminEventDetailPage() {
       return;
     }
 
-    setSending(true);
     try {
       const res = await emailsApi.sendToEvent(eventId, {
         templateSlug: selectedTemplate,
@@ -432,14 +431,12 @@ export default function AdminEventDetailPage() {
       });
 
       if (res.success) {
-        toast.success(`Email sent to ${res.sentCount} recipients`);
+        toast.success(`${res.queuedCount} email(s) are being sent in the background.`);
       } else {
-        toast.error(`Sent: ${res.sentCount}, Failed: ${res.failedCount}`);
+        toast.error(res.error || 'Failed to queue emails');
       }
     } catch (error: any) {
       toast.error(error.message || 'Failed to send emails');
-    } finally {
-      setSending(false);
     }
   };