XF2 Basic Jack - Payment Refund 1.0.0

This add-on adds two capabilities missing from XenForo's payment system:

1. Admin-initiated refunds — Issue full or partial refunds directly from the admin panel instead of logging into the provider dashboards (e.g. Stripe, PayPal, etc).
2. Smart partial refund handling — Fixes a core XenForo behavior where partial refunds (e.g., a $15 discount adjustment on a $30 purchase) downgrades the user vs only refunds. Partial refunds now log as informational events without triggering a reversal. Full refunds continue to auto-downgrade as before.

Tactically put, the add-on does four things:

1. Provides a refund framework for providers - Defines the supportsRefunds() / refund() interface that any payment provider can implement, plus the payment_refund_complete event for purchasable add-ons to listen to.
2. Provider Updates - Extends both Stripe and PayPal REST providers with their refund API calls, webhook deduplication, and partial refund handling.
3. Admin UI - Adds the "Issue Refund" button and form to payment provider log entries.
4. User Upgrade handling - Since user upgrades are native to XenForo, this add-on tries to complete partial vs full refund handling for User Upgrade purchases. The behavior of any refund amount triggering a downgrade has been changed to support partial refund scenarios. However, if you want to do a partial refund and downgrade the user, the UX provides that capability.

Other add-on developers would be responsible for implementing refund support in their own payment providers (via the new supportsRefunds() / refund()) that this addon provides) and their own purchasable types (via the payment_refund_complete code event).

1. Admin navigates to Logs → Payment Provider → [specific payment entry]
2. Clicks "Issue Refund" (only visible for providers that support refunds)
3. Enters refund amount (pre-filled with remaining refundable balance)
4. For user upgrades: option to reverse the purchase (downgrade user)
5. For other purchasable types: the refund is processed with the provider, and the owning add-on handles reversal via the payment_refund_complete code event
6. The add-on calls the provider's refund API, logs the result, and tracks the cumulative refunded amount