Phase 5 Xendit: Stages 1-7 (XENDIT_ENABLED=false; Stage 8 pending creds)

Backend
- payment_sessions → payment_requests rename across DB schema + 29 files
- payment.service.js becomes product-agnostic owner: EventEmitter +
  Xendit wrapper + requestPayment / confirmPayment public API; legacy
  aliases retained for existing chat callers
- Webhook handler at POST /api/shared/payment/webhooks/xendit, with
  constant-time token verification (8 vitest cases)
- Server-driven pairing: payment.service emits
  payment_request.confirmed → pairing subscriber starts the blast.
  Legacy POST /chat/request still works during the cutover.
- Reconciliation sweeper extended (re-emits events for confirmed rows
  with no chat session)
- SIGTERM drain + startup reconciliation pass in server.js

Customer app
- waiting_payment_screen opens xendit_invoice_url via
  LaunchMode.inAppBrowserView
- searching / no-bestie / targeted-waiting / pairing-notifier updated
  to consume the new payment_request_id contract
- pending_payments_provider + bestie-unavailable dialog migrated

Dev / testing
- XENDIT_ENABLED=false is the safe default; .env.example documents the
  four new vars
- backend/.dev/xendit-fake-webhook.sh exercises the handler without
  ngrok
- 90/92 backend tests pass (two pre-existing session-timer flakes,
  unrelated); client_app analyzer clean
- requirement/phase5-xendit-plan.md is the canonical reference

Stage 8 (live E2E) blocked on Xendit test-mode keys. The dashboard's
single-webhook-URL constraint will be worked around via a self-poll
script next session.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

client_app/lib/features/chat_tab/providers/pending_payments_provider.dart

@@ -4,7 +4,7 @@ import '../../../core/auth/auth_notifier.dart';
 
 /// One row in the Chat Tab > Pembayaran sub-tab.
 ///
-/// Mirrors the response of `GET /api/client/payment-sessions/pending`. A row
+/// Mirrors the response of `GET /api/client/payment-requests/pending`. A row
 /// is either an initial-session payment (`isExtension == false`) — for which
 /// mitra info is only present in the targeted "Curhat lagi" flow — or an
 /// extension payment (`isExtension == true`) — mitra info resolved by the
@@ -80,7 +80,7 @@ final pendingPaymentsProvider =
   if (customerId == null) return PendingPaymentsData.empty;
   final api = ref.read(apiClientProvider);
   final response =
-      await api.get('/api/client/payment-sessions/pending');
+      await api.get('/api/client/payment-requests/pending');
   final data = response['data'] as Map<String, dynamic>? ?? const {};
   final items = (data['items'] as List<dynamic>? ?? [])
       .cast<Map<String, dynamic>>()