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/core/constants.dart

@@ -64,7 +64,7 @@ class ExtensionStatus {
   ExtensionStatus._();
 }
 
-/// Session mode — chat or voice call. Mirrors backend `payment_sessions.mode`
+/// Session mode — chat or voice call. Mirrors backend `payment_requests.mode`
 /// (added in Phase 4 stage 1). A `call` session is functionally a chat with a
 /// "voice call" badge and (eventually) a Meet link the mitra pastes manually;
 /// no real audio transport is built yet.
@@ -153,7 +153,7 @@ enum PairingFailureCause {
   targetedMitraOffline('targeted_mitra_offline'),
   targetedMitraRejected('targeted_mitra_rejected'),
   targetedMitraTimeout('targeted_mitra_timeout'),
-  paymentSessionExpired('payment_session_expired'),
+  paymentSessionExpired('payment_request_expired'),
   customerCancelled('customer_cancelled'),
   unknown('unknown');
 
@@ -165,13 +165,13 @@ enum PairingFailureCause {
 }
 
 /// Payment session lifecycle. Mirror of backend
-/// `PaymentSessionStatus`.
-class PaymentSessionStatus {
+/// `PaymentRequestStatus`.
+class PaymentRequestStatus {
   static const pending = 'pending';
   static const confirmed = 'confirmed';
   static const consumed = 'consumed';
-  static const failedPairing = 'failed_pairing';
+  static const failedPairing = 'failed_delivery';
   static const abandoned = 'abandoned';
   static const expired = 'expired';
-  PaymentSessionStatus._();
+  PaymentRequestStatus._();
 }