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>

control_center/src/core/constants.js

@@ -8,7 +8,7 @@ export const PairingFailureCause = Object.freeze({
   TARGETED_MITRA_OFFLINE: 'targeted_mitra_offline',
   TARGETED_MITRA_REJECTED: 'targeted_mitra_rejected',
   TARGETED_MITRA_TIMEOUT: 'targeted_mitra_timeout',
-  PAYMENT_SESSION_EXPIRED: 'payment_session_expired',
+  PAYMENT_REQUEST_EXPIRED: 'payment_request_expired',
   CUSTOMER_CANCELLED: 'customer_cancelled',
   EXTENSION_REJECTED: 'extension_rejected',
   EXTENSION_SAFEGUARD_TRIPPED: 'extension_safeguard_tripped',
@@ -20,7 +20,7 @@ export const PairingFailureCauseLabel = Object.freeze({
   [PairingFailureCause.TARGETED_MITRA_OFFLINE]: 'Targeted mitra offline',
   [PairingFailureCause.TARGETED_MITRA_REJECTED]: 'Targeted mitra rejected',
   [PairingFailureCause.TARGETED_MITRA_TIMEOUT]: 'Targeted mitra timeout',
-  [PairingFailureCause.PAYMENT_SESSION_EXPIRED]: 'Payment session expired',
+  [PairingFailureCause.PAYMENT_REQUEST_EXPIRED]: 'Payment session expired',
   [PairingFailureCause.CUSTOMER_CANCELLED]: 'Customer cancelled',
   [PairingFailureCause.EXTENSION_REJECTED]: 'Extension rejected',
   [PairingFailureCause.EXTENSION_SAFEGUARD_TRIPPED]: 'Extension safeguard tripped',