docusign-paypal-extension-app — DocuSign extension scaffold for PayPal payments.

Endpoints DocuSign will call (POST):
  /api/v1/payments/capture-transaction      — required
  /api/v1/payments/void-transaction         — required
  /api/v1/payments/authorize-transaction    — optional capability
  /api/v1/payments/list-payment-methods     — optional capability
  /api/v1/payments/get-products-list        — optional capability

Recurring schedules (ours, not part of the Docusign contract):
  POST /api/v1/recurring/schedules              — start a schedule on a vaulted method
  GET  /api/v1/recurring/schedules              — list schedules
  GET  /api/v1/recurring/schedules/{id}         — schedule + attempt history
  POST /api/v1/recurring/schedules/{id}/cancel  — stop a schedule
  POST /api/v1/recurring/run                    — run the due-charge tick now

OAuth (DocuSign ↔ this service):
  GET  /oauth/authorize  — install entry; redirects to PayPal onboarding
  POST /oauth/token      — auth-code → JWT access_token exchange
  GET  /oauth/userinfo   — returns merchant identity from the JWT

PayPal Connected Path:
  GET  /paypal/onboard/return — PayPal redirects merchant here after onboarding;
                               we redirect back to DocuSign with the auth code

Discovery / docs:
  GET /api/docs                — interactive API docs (Scalar)
  GET /api/docs/spec.json      — OpenAPI 3 spec
  GET /manifest.json           — the extension manifest
  GET /checkout                — embedded checkout iframe placeholder

Self-contained — all PayPal calls go through src/paypal/* using the partner
OAuth token + PayPal-Auth-Assertion JWT. See README.md for architecture.