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>

backend/package-lock.json

@@ -22,6 +22,7 @@
         "jwks-rsa": "^3.2.2",
         "pg": "^8.12.0",
         "postgres": "^3.4.4",
+        "xendit-node": "^7.0.0",
         "zod": "^3.23.8"
       },
       "devDependencies": {
@@ -4829,6 +4830,14 @@
         }
       }
     },
+    "node_modules/xendit-node": {
+      "version": "7.0.0",
+      "resolved": "https://registry.npmjs.org/xendit-node/-/xendit-node-7.0.0.tgz",
+      "integrity": "sha512-atsCQ9femoWLu+hU5rY0TZ+ZhqSYbpTtABZN7rGMhhBh1xLhTuY1rgLfoaXzFiRt3eOQRqsew78wPUcQn1QckA==",
+      "engines": {
+        "node": ">=18.0.0"
+      }
+    },
     "node_modules/xtend": {
       "version": "4.0.2",
       "resolved": "https://registry.npmjs.org/xtend/-/xtend-4.0.2.tgz",