feat(email): hold relationship mail to business hours

queueEmail gains options.respectBusinessHours: snaps the send time to the
next open business-hours block (from now), only deferring when it actually
falls outside hours. Applied to dunning reminders + gallery-expiry warnings;
transactional/admin-initiated mail stays immediate. No-op until business
hours are configured.

backend/src/services/invoiceService.js

@@ -2650,7 +2650,9 @@ async function applyReminder(invoice, lineItems, level, adminId) {
       contentPath: pdfPath,
       contentType: 'application/pdf',
     }],
-  });
+  // Dunning reminders are relationship mail — hold to business hours so
+  // the customer isn't pinged overnight (no-op unless hours configured).
+  }, { respectBusinessHours: true });
 
   try {
     await logActivity('invoice_reminder_sent', { invoiceId: invoice.id, level, lateFeeMinor },