feat: pay allowance requests on-chain once fees count against the limits #1592

Refs:

• feat: add allowances for automatic paykit payments #1340
• feat: add allowances for automatic paykit payments #1340 (comment)

What happened?

Automatic on-chain allowance payments spend a mining fee that no allowance limit counts. The SDK's allowance ledger records the request amount only, since it requires the paid amount to equal the request, and the per-payment and monthly checks in PaykitAllowanceExecutor use the same amount. This PR adds a stopgap: an automatic on-chain payment whose fee exceeds the amount stays manual. A fee up to the amount still falls outside the limits, so a contact that publishes only on-chain endpoints can make the wallet spend up to twice what the allowance shows as paid.

Expected behavior

The total an allowance can make the wallet spend, fees included, stays within what the user approved: either fees count against the per-payment and monthly limits, or automatic payments use only Lightning.

Steps to Reproduce

1. Grant a contact $5 a payment and $10 a month.
2. The contact publishes only an on-chain endpoint and sends many small requests.
3. Each request is paid on-chain with a Medium-rate fee; the detail sheet shows only the amounts.

Bitkit Version

Unreleased: allowances branch of #1340.

Reproducibility

From the code path; not yet run on a device.