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/emails/page.tsx

@@ -189,7 +189,6 @@ export default function AdminEmailsPage() {
       return;
     }
     
-    setSending(true);
     try {
       const res = await emailsApi.sendToEvent(composeForm.eventId, {
         templateSlug: composeForm.templateSlug,
@@ -197,20 +196,15 @@ export default function AdminEmailsPage() {
         customVariables: composeForm.customBody ? { customMessage: composeForm.customBody } : undefined,
       });
       
-      if (res.success || res.sentCount > 0) {
-        toast.success(`Sent ${res.sentCount} emails successfully`);
-        if (res.failedCount > 0) {
-          toast.error(`${res.failedCount} emails failed`);
-        }
+      if (res.success) {
+        toast.success(`${res.queuedCount} email(s) are being sent in the background.`);
         clearDraft();
         setShowRecipientPreview(false);
       } else {
-        toast.error('Failed to send emails');
+        toast.error(res.error || 'Failed to queue emails');
       }
     } catch (error: any) {
       toast.error(error.message || 'Failed to send emails');
-    } finally {
-      setSending(false);
     }
   };