Phase 3 scaffold: chat engine (WebSocket, FCM, pricing, timer, extension, history)

- Backend: WebSocket plugin, chat/pricing/timer/extension/closure/notification services
- Client app: ChatBloc, pricing dialog, chat screen with message status, extension/goodbye flow, history
- Mitra app: MitraChatBloc, ExtensionBloc, chat screen, extension accept/reject, history
- Control center: free trial, extension timeout, early end config toggles
- DB migration: chat_messages, session_closures, session_extensions, customer_transactions tables

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/services/notification.service.js

@@ -0,0 +1,52 @@
+import admin from 'firebase-admin'
+import { getDb } from '../db/client.js'
+
+const sql = getDb()
+
+export const registerDeviceToken = async (userType, userId, fcmToken) => {
+  const table = userType === 'customer' ? 'customers' : 'mitras'
+  await sql`
+    UPDATE ${sql(table)}
+    SET fcm_token = ${fcmToken}
+    WHERE id = ${userId}
+  `
+}
+
+export const sendPushNotification = async (recipientType, recipientId, { title, body, data = {} }) => {
+  const table = recipientType === 'customer' ? 'customers' : 'mitras'
+  const [user] = await sql`
+    SELECT fcm_token FROM ${sql(table)} WHERE id = ${recipientId}
+  `
+
+  if (!user?.fcm_token) return false
+
+  try {
+    await admin.messaging().send({
+      token: user.fcm_token,
+      notification: { title, body },
+      data: {
+        ...Object.fromEntries(
+          Object.entries(data).map(([k, v]) => [k, String(v)])
+        ),
+        click_action: 'FLUTTER_NOTIFICATION_CLICK',
+      },
+      android: {
+        priority: 'high',
+        notification: { channelId: 'chat_messages' },
+      },
+      apns: {
+        payload: {
+          aps: { sound: 'default', badge: 1 },
+        },
+      },
+    })
+    return true
+  } catch (err) {
+    console.error(`[FCM] Failed to send to ${recipientType}:${recipientId}:`, err.message)
+    // Clear invalid token
+    if (err.code === 'messaging/registration-token-not-registered') {
+      await sql`UPDATE ${sql(table)} SET fcm_token = NULL WHERE id = ${recipientId}`
+    }
+    return false
+  }
+}