Instant Donation Refunds

For refunded donations, we are currently waiting for an `FS Main -> FS Op` transfer to remove the refunded donation money from the event. This can take a day — meaning the org has access to money they don't own.

### Solution:
- Add pending transaction to the hcb code's group when a refund is issued
- Allow the pending transaction to modify `amount_cents` of hcb code (some weird logic needs to happen here)
- At the moment, hcb code calculates `amount_cents` using only CT — meaning the added PT won't affect `amount_cents`
https://github.com/hackclub/bank/blob/a7ae664fd1241472ee03521edbe42510b2bf801c/app/models/hcb_code.rb#L34-L40
- Settle the pending transaction once the refund transfer arrives
- https://github.com/hackclub/bank/blob/a7ae664fd1241472ee03521edbe42510b2bf801c/app/services/pending_event_mapping_engine/settle/donation_hcb_code.rb

Start with app/models/hcb_code.rb at the linked amount_cents logic, then read app/services/pending_event_mapping_engine/settle/donation_hcb_code.rb. Trace how refunded donations and the FS Main -> FS Op transfer are currently handled. Done means a pending transaction is added and reflected in the HCB code, then settled when the refund transfer arrives.

Written by the indexing model from the issue text.