Pay fiat-settled referral commission by converting to sats

[v0l]

Closes #312.

A fiat-settled balance is quoted against BTC at send time, paid over Lightning, and recorded with both sides (referral/mod.rs:242 for the pass, converted_payout for the row).

• Off by default: min-fiat-payout-sats under [referral], None disables — the same shape as the existing BTC minimum, and the referrer's own payout_threshold still applies on top.
• The threshold is judged on the converted value, so one setting means the same thing whatever the currency. That answers the issue's open question — no second unit to keep in sync.
• The settled amount is derived back from the rounded-to-whole-sats transfer, so the sub-sat remainder stays owed instead of being discharged for free. The routing fee is charged at the same quote rather than at read time.
• Netting is per settled currency (settled_in), so a EUR payout sent as BTC leaves the BTC balance alone — the invariant Record both sides of a converted referral payout #311 established.

Scope: Lightning/NWC referrers only. An on-chain payout batches referrers into one transaction and splits its fee across them in sats; charging that share against a fiat balance means converting each share back, which is a different change. On-chain referrers keep accruing fiat for manual payout, and the config comment says so. Happy to file that as a follow-up.

Tests: unit coverage of the conversion, the threshold, the per-currency grouping and the netting. No e2e — nothing here is reachable over HTTP, and the payout itself needs a real LNURL endpoint.

Originating channel: Buzz #721a8a6c-c36c-5fa1-ae07-822ddc8567c5