[Invoices] sync_remote! can overwrite the record of a completed payment

## Problem

`Invoice#set_fields_from_stripe_invoice` unconditionally copies `amount_paid`, `amount_due`, `amount_remaining`, `status`, `subtotal`, and `total` from the current remote Stripe invoice. There's no guard for invoices HCB has already transitioned to `paid_v2` and paid out.

So if a Stripe invoice changes state out of band after being paid (e.g. voided from the Stripe Dashboard), the next sync overwrites the local payment fields. The invoice stays `paid_v2` with a real `InvoicePayout`, but ends up recording `amount_paid = 0` and a remote status of `void`.

Example: invoice `4881`.

## Impact

No money is affected — payouts and fee reimbursements derive from the charge's balance transaction (`payout_creation_balance_net` / `payout_creation_balance_stripe_fee`), not from these columns. The damage is to the record: the invoice no longer reflects what was actually collected, and anything reading `amount_paid` / `amount_remaining` for it is wrong.

## Where it can fire unexpectedly

`invoices#hosted` and `invoices#pdf` both call `sync_remote!` (which ends in `save!`) on a GET. Viewing the hosted invoice or the PDF is enough to rewrite these columns.

## Possible directions

- Don't overwrite payment fields once an invoice is `paid_v2` and/or has a payout.
- Report the divergence instead of silently overwriting it.
- Reconsider writing to the database from those GET actions.

---
_{Filed by Claude at @garyhtou's request.}

Start with Invoice#set_fields_from_stripe_invoice and trace the sync_remote! calls from invoices#hosted and invoices#pdf, paying attention to their save! behavior on GET. Use invoice 4881 as the reported example and define done as preventing a paid_v2 invoice with an InvoicePayout from losing its recorded payment fields when the remote invoice changes state.

Written by the indexing model from the issue text.