Add pay gem and payments/allocations/refunds

[jmilljr24]

Closes [link an issue or remove this line]

What is the goal of this PR and why is this important?

EventRegistations (and more models in the future) will require payments. This add the initial framework to handle payments.

How did you approach the change?

• Add Payments, Allocations, Refunds models.
• Add pay gem to add payments via stripe
• Payment types: Cash, Check, ExternalProcessor (anything through pay gem)

Allocations - a payment can be allocated to an event_reg.
Payments - a payer can be a person or organization and can add funds via payment types.

Payments can be refunded via the Refund model. Allocations can be reverted (original allocation record doesn't change. A new Allocation record allocates the funds and references the original allocation.

I decided to add an amount_remaining_cents on a Payment. This is the source of truth for was funds are available that have not been allocated or refunded. Checking based of a all allocations and refunds is possible but this prevents race conditions as well as any data that may be out of sync with stripe via webhooks.

UI Testing Checklist

Anything else to add?

The video is not in any special order. Just an overview of the features. UI styling was a low priority since this is such a big change.
Shown is the payments index filtering, Payment creation (without allocating), Allocating a payment, Partial payment Refund, Allocations on a persons event_reg (some examples show an allocation that has been reverted (grayed out with another line item showing the revert in red)

screenrecording-2026-04-09_16-49-42.mp4