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/widgets/bestie_unavailable_dialog.dart

@@ -23,7 +23,7 @@ import '../../support/widgets/tanya_admin_sheet.dart';
 ///   payment session exists yet, so the "cari bestie lain" CTA resets the
 ///   payment draft and pushes `/payment/entry` for a fresh blast-payment
 ///   flow. This branch never calls [Pairing.fallbackToBlast] because there's
-///   no `paymentSessionId` to attach to.
+///   no `paymentRequestId` to attach to.
 /// - [BestieOfflineVariant.new_] — the customer triggered a general blast
 ///   that bottomed out (no online besties). No fallback button; just a
 ///   ghost `tanya admin` and a `kembali ke home` exit.
@@ -35,14 +35,14 @@ enum BestieOfflineVariant { returning, prePayReturning, new_ }
 class BestieOfflinePopup extends ConsumerWidget {
   final BestieOfflineVariant variant;
   final String mitraName;
-  final String? paymentSessionId;
+  final String? paymentRequestId;
   final TopicSensitivity? topicSensitivity;
 
   const BestieOfflinePopup({
     super.key,
     required this.variant,
     required this.mitraName,
-    this.paymentSessionId,
+    this.paymentRequestId,
     this.topicSensitivity,
   });
 
@@ -50,7 +50,7 @@ class BestieOfflinePopup extends ConsumerWidget {
     BuildContext context, {
     required BestieOfflineVariant variant,
     required String mitraName,
-    String? paymentSessionId,
+    String? paymentRequestId,
     TopicSensitivity? topicSensitivity,
   }) {
     return showDialog<void>(
@@ -60,7 +60,7 @@ class BestieOfflinePopup extends ConsumerWidget {
       builder: (_) => BestieOfflinePopup(
         variant: variant,
         mitraName: mitraName,
-        paymentSessionId: paymentSessionId,
+        paymentRequestId: paymentRequestId,
         topicSensitivity: topicSensitivity,
       ),
     );
@@ -83,7 +83,7 @@ class BestieOfflinePopup extends ConsumerWidget {
 
     final canFallbackToBlast = isReturning &&
         hasOtherAvailable &&
-        paymentSessionId != null &&
+        paymentRequestId != null &&
         topicSensitivity != null;
 
     return Dialog(
@@ -145,7 +145,7 @@ class BestieOfflinePopup extends ConsumerWidget {
                   Navigator.of(context).pop();
                   // ignore: discarded_futures
                   ref.read(pairingProvider.notifier).fallbackToBlast(
-                        paymentSessionId: paymentSessionId!,
+                        paymentRequestId: paymentRequestId!,
                         topicSensitivity: topicSensitivity!,
                       );
                 },