Phase 3.1 WS2: Backend FCM fallback, ping config, unread API

- Add require_mitra_ping + mitra_ping_interval_seconds config keys (migration)
- Add getMitraPingConfig/setMitraPingConfig to config service
- Add GET/PATCH /internal/config/mitra-ping routes for control center
- Update mitra status service: honor ping config in auto-offline sweep,
  include ping config in GET /api/mitra/status response
- Enhance pairing FCM payload with action: 'open_accept' for deep-link
- Add FCM fallback to closure.service (initiateEarlyEnd, completeSession)
- Add FCM fallback to session-timer.service (onSessionExpired)
- Add unread count queries (getActiveSessionByCustomerWithUnread,
  getActiveSessionsByMitraWithUnread)
- Add GET /api/client/chat/session/active-with-unread route
- Add GET /api/mitra/chat-requests/sessions/active-with-unread route

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

backend/src/services/session.service.js

@@ -1,6 +1,6 @@
 import { getDb } from '../db/client.js'
 import { publish } from '../plugins/valkey.js'
-import { UserType, SessionStatus, WsMessage } from '../constants.js'
+import { UserType, SessionStatus, MessageStatus, WsMessage } from '../constants.js'
 
 const sql = getDb()
 
@@ -155,6 +155,42 @@ export const getSessionById = async (sessionId) => {
   return session
 }
 
+// --- Phase 3.1: Unread counts ---
+
+export const getActiveSessionByCustomerWithUnread = async (customerId) => {
+  const [session] = await sql`
+    SELECT cs.id, cs.customer_id, cs.mitra_id, cs.status, cs.created_at, cs.paired_at,
+      cs.duration_minutes, cs.price, cs.is_free_trial, cs.expires_at, cs.extended_minutes,
+      m.display_name AS mitra_display_name,
+      (SELECT COUNT(*) FROM chat_messages cm
+       WHERE cm.session_id = cs.id AND cm.sender_type = ${UserType.MITRA}
+       AND cm.status IN (${MessageStatus.SENT}, ${MessageStatus.DELIVERED}))::int AS unread_count
+    FROM chat_sessions cs
+    LEFT JOIN mitras m ON m.id = cs.mitra_id
+    WHERE cs.customer_id = ${customerId}
+      AND cs.status IN (${SessionStatus.ACTIVE}, ${SessionStatus.PENDING_PAYMENT}, ${SessionStatus.EXTENDING}, ${SessionStatus.CLOSING})
+    ORDER BY cs.created_at DESC LIMIT 1
+  `
+  return session
+}
+
+export const getActiveSessionsByMitraWithUnread = async (mitraId) => {
+  const sessions = await sql`
+    SELECT cs.id, cs.customer_id, cs.status, cs.created_at, cs.paired_at,
+      cs.duration_minutes, cs.expires_at, cs.extended_minutes,
+      c.display_name AS customer_display_name,
+      (SELECT COUNT(*) FROM chat_messages cm
+       WHERE cm.session_id = cs.id AND cm.sender_type = ${UserType.CUSTOMER}
+       AND cm.status IN (${MessageStatus.SENT}, ${MessageStatus.DELIVERED}))::int AS unread_count
+    FROM chat_sessions cs
+    INNER JOIN customers c ON c.id = cs.customer_id
+    WHERE cs.mitra_id = ${mitraId}
+      AND cs.status IN (${SessionStatus.ACTIVE}, ${SessionStatus.PENDING_PAYMENT}, ${SessionStatus.EXTENDING}, ${SessionStatus.CLOSING})
+    ORDER BY cs.created_at DESC
+  `
+  return sessions
+}
+
 export const getCustomerHistory = async (customerId, { page = 1, limit = 20 } = {}) => {
   const offset = (page - 1) * limit
   const items = await sql`