fix: batch email sending in message system to avoid overloading SMTP

Messages were sent in a tight for-loop with no throttling. Now uses
sendBatchNotifications (10/batch, 150ms per email, 500ms between
batches) and fires in the background so the admin gets instant response.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/server/routers/message.ts

@@ -3,6 +3,8 @@ import { TRPCError } from '@trpc/server'
 import { router, protectedProcedure, adminProcedure } from '../trpc'
 import { logAudit } from '@/server/utils/audit'
 import { sendStyledNotificationEmail, getEmailPreviewHtml } from '@/lib/email'
+import { sendBatchNotifications } from '../services/notification-sender'
+import type { NotificationItem } from '../services/notification-sender'
 
 export const messageRouter = router({
   /**
@@ -68,7 +70,7 @@ export const messageRouter = router({
         },
       })
 
-      // If not scheduled, deliver immediately for EMAIL channel
+      // If not scheduled, deliver immediately for EMAIL channel (batched to avoid overloading SMTP)
       if (!isScheduled && input.deliveryChannels.includes('EMAIL')) {
         const users = await ctx.prisma.user.findMany({
           where: { id: { in: recipientUserIds } },
@@ -77,23 +79,25 @@ export const messageRouter = router({
 
         const baseUrl = process.env.NEXTAUTH_URL || 'https://portal.monaco-opc.com'
 
-        for (const user of users) {
-          try {
-            await sendStyledNotificationEmail(
-              user.email,
-              user.name || '',
-              'MESSAGE',
-              {
-                name: user.name || undefined,
-                title: input.subject,
-                message: input.body,
-                linkUrl: `${baseUrl}/messages`,
-              }
-            )
-          } catch (error) {
-            console.error(`[Message] Failed to send email to ${user.email}:`, error)
-          }
-        }
+        const items: NotificationItem[] = users.map((user) => ({
+          email: user.email,
+          name: user.name || '',
+          type: 'MESSAGE',
+          userId: user.id,
+          context: {
+            name: user.name || undefined,
+            title: input.subject,
+            message: input.body,
+            linkUrl: `${baseUrl}/messages`,
+          },
+        }))
+
+        // Fire-and-forget: batch send in background so the mutation returns quickly
+        sendBatchNotifications(items).then((result) => {
+          console.log(`[Message] Batch ${result.batchId}: ${result.sent} sent, ${result.failed} failed`)
+        }).catch((err) => {
+          console.error('[Message] Batch send error:', err)
+        })
       }
 
       try {