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/config.service.js

@@ -82,6 +82,35 @@ export const getEarlyEndConfig = async () => {
   }
 }
 
+// --- Phase 3.1: Mitra Ping Config ---
+
+export const getMitraPingConfig = async () => {
+  const [requireRow] = await sql`SELECT value FROM app_config WHERE key = 'require_mitra_ping'`
+  const [intervalRow] = await sql`SELECT value FROM app_config WHERE key = 'mitra_ping_interval_seconds'`
+  return {
+    require_ping: requireRow?.value?.value ?? true,
+    ping_interval_seconds: intervalRow?.value?.value ?? 15,
+  }
+}
+
+export const setMitraPingConfig = async ({ require_ping, ping_interval_seconds }) => {
+  if (require_ping !== undefined) {
+    await sql`
+      INSERT INTO app_config (key, value, updated_at)
+      VALUES ('require_mitra_ping', ${sql.json({ value: require_ping })}, NOW())
+      ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value, updated_at = NOW()
+    `
+  }
+  if (ping_interval_seconds !== undefined) {
+    await sql`
+      INSERT INTO app_config (key, value, updated_at)
+      VALUES ('mitra_ping_interval_seconds', ${sql.json({ value: ping_interval_seconds })}, NOW())
+      ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value, updated_at = NOW()
+    `
+  }
+  return getMitraPingConfig()
+}
+
 export const setEarlyEndConfig = async ({ mitra_enabled, customer_enabled }) => {
   if (mitra_enabled !== undefined) {
     await sql`