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/screens/searching_screen.dart

@@ -70,7 +70,7 @@ class _SearchingScreenState extends ConsumerState<SearchingScreen> {
           if (draft.targetedMitraId != null) {
             // ignore: discarded_futures
             ref.read(pairingProvider.notifier).startTargetedSearch(
-                  paymentSessionId: draft.paymentId!,
+                  paymentRequestId: draft.paymentId!,
                   mitraId: draft.targetedMitraId!,
                   mitraName: draft.targetedMitraName ?? 'Bestie',
                   topicSensitivity: draft.topicSensitivity,
@@ -80,7 +80,7 @@ class _SearchingScreenState extends ConsumerState<SearchingScreen> {
           }
           // ignore: discarded_futures
           ref.read(pairingProvider.notifier).startSearch(
-                paymentSessionId: draft.paymentId!,
+                paymentRequestId: draft.paymentId!,
                 topicSensitivity: draft.topicSensitivity,
               );
         }
@@ -117,7 +117,7 @@ class _SearchingScreenState extends ConsumerState<SearchingScreen> {
         context,
         variant: BestieOfflineVariant.returning,
         mitraName: next.mitraName,
-        paymentSessionId: next.paymentSessionId,
+        paymentRequestId: next.paymentRequestId,
         topicSensitivity: next.topicSensitivity,
       ).then((_) {
         if (mounted) _unavailableDialogShown = false;