Phase 3.7: paid pairing flow + returning chat + extension flip

- Backend: payment_sessions + pairing_failures tables; payment.service.js
  and pairing-failure.service.js (new); rewritten pairing.service.js
  (payment-gated blast + targeted "Curhat lagi" + cancel + fallback);
  rewritten extension.service.js (data-driven auto-approve with offline
  safeguard, charge-at-approval); pricing.service.js (extension tiers
  without free trial); mitra-status.service.js (countAvailableMitras
  cached path); 60s sweeper for stale payment sessions
- Backend routes: client.payment.routes, client.mitra-availability.routes,
  internal/failed-pairings.routes; client.chat.routes rewritten for
  payment-gated start + /returning + /cancel + /fallback-to-blast;
  internal/config.routes adds 4 new keys with Valkey invalidate publish
- client_app: mitra-availability poll, payment screen + notifier, pairing
  notifier rewrite (PairingTargetedWaiting + PairingFailed states),
  targeted-waiting overlay + bestie-unavailable dialog, "Curhat lagi"
  CTA, failed-pairing terminal, extension via payment-session
- mitra_app: PairingRequestType enum, returning-chat 20s countdown
  auto-dismiss, extension card "otomatis disetujui" copy
- control_center: 4 new config rows in Settings, Failed Pairings page
  (filter + paginate + action menu), sidebar + route registered
- Test infrastructure: Vitest backend (7/7 pass), Playwright CC (4/4
  pass), Maestro mobile scaffold (CLI install pending)
- Bugs found via Playwright + fixed: LoginPage labels not associated
  with inputs (a11y); backend internal CORS missing PATCH/PUT/DELETE
  in allow-methods (silent settings breakage in browsers since Stage 4)
- Docs: phase3.7.md PRD, phase3.7-plan.md, phase3.7-questions.md (Q&A),
  phase3.7-testing.md (E2E checklist), phase3.7-test-run-2026-05-03.md
  (today's run results)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/lib/core/constants.dart

@@ -9,6 +9,17 @@ String formatCountdown(int totalSeconds) {
   return '${minutes}m ${seconds}d';
 }
 
+/// Format an integer rupiah amount with dot thousand-separators: 1234567 → "Rp 1.234.567".
+String formatRupiah(int amount) {
+  final str = amount.toString();
+  final buffer = StringBuffer();
+  for (var i = 0; i < str.length; i++) {
+    if (i > 0 && (str.length - i) % 3 == 0) buffer.write('.');
+    buffer.write(str[i]);
+  }
+  return 'Rp $buffer';
+}
+
 /// User types
 class UserType {
   static const customer = 'customer';
@@ -105,5 +116,44 @@ class WsMessage {
   // Early end
   static const earlyEnd = 'early_end';
 
+  // Returning-chat (intermediate failures — payment stays confirmed)
+  static const returningChatTimeout = 'returning_chat_timeout';
+  static const returningChatRejected = 'returning_chat_rejected';
+
+  // Terminal pairing failure on a confirmed payment session
+  static const pairingFailed = 'pairing_failed';
+
   WsMessage._();
 }
+
+/// Pairing-failure cause tags. Mirror of backend
+/// `PairingFailureCause` (see backend/src/constants.js). Use for both routing
+/// (terminal vs. intermediate) and surfacing copy on the failed-pairing screen.
+enum PairingFailureCause {
+  noMitraAvailable('no_mitra_available'),
+  allMitrasRejected('all_mitras_rejected'),
+  targetedMitraOffline('targeted_mitra_offline'),
+  targetedMitraRejected('targeted_mitra_rejected'),
+  targetedMitraTimeout('targeted_mitra_timeout'),
+  paymentSessionExpired('payment_session_expired'),
+  customerCancelled('customer_cancelled'),
+  unknown('unknown');
+
+  final String value;
+  const PairingFailureCause(this.value);
+
+  static PairingFailureCause fromString(String? v) =>
+      values.firstWhere((e) => e.value == v, orElse: () => PairingFailureCause.unknown);
+}
+
+/// Payment session lifecycle. Mirror of backend
+/// `PaymentSessionStatus`.
+class PaymentSessionStatus {
+  static const pending = 'pending';
+  static const confirmed = 'confirmed';
+  static const consumed = 'consumed';
+  static const failedPairing = 'failed_pairing';
+  static const abandoned = 'abandoned';
+  static const expired = 'expired';
+  PaymentSessionStatus._();
+}