PayPal Payouts

- SECURITY: Webhook signature verification now fails CLOSED. Previously, if no Webhook ID was configured, verification returned success, allowing an attacker to forge PAYMENT.PAYOUTS events. It now rejects the webhook. ACTION REQUIRED: configure the Webhook ID to continue receiving PayPal webhook status updates.
- SECURITY: Webhook replay protection. The signed paypal-transmission-time is now validated for freshness (5-minute window), so a captured valid webhook can no longer be replayed indefinitely.
- SECURITY: The batch sender_batch_id is now derived deterministically from the payout references instead of a random UUID. A retry after a lost response now reuses the same batch id, so PayPal returns BATCH_NOT_UNIQUE instead of paying every recipient twice.
- Payout results now correspond 1:1 to the input requests: recipients skipped for a missing PayPal email are returned as explicit failures instead of being reported as pending payouts that were never submitted.
- Partial batch cancellations are no longer reported as fully cancelled; CANCELLED is returned only when every unclaimed item was cancelled, otherwise the result is a failure with a count.
- Amounts are formatted to PayPal's required precision per currency (0 decimals for HUF/JPY/TWD, 2 otherwise) instead of sending the raw Decimal string.
- Added the documented CANCELED batch status to the status map (was falling back to pending), and estimated fees are rounded to the currency's minor units.

- Initial release
- Batch payouts API support (up to 15,000 per batch)
- OAuth 2.0 authentication with token caching
- Webhook signature verification
- Fee estimation
- Sandbox and Production environment support