feat: name the applied rate in the payout memo

A back-dated payout's sat figure is unexplainable on its own — employee_2's
August backfill paid 18,353 sat and 14,794 sat for the same ten euro, and
only the rate says why. Both wallets now show it:

    dev retainer — 2026-08-01 · 10 EUR @ 54,487 EUR/BTC
    dev retainer — 2026-08-29 · 10 EUR @ 67,596.6 EUR/BTC

This required moving the `current` mode conversion into payroll as well.
Previously that mode handed create_invoice a fiat amount and let LNbits
convert, so the rate only became knowable by reading it back off the
resulting invoice — after the memo had already been fixed. Now every mode
resolves its rate before invoicing and the memo is accurate in all three,
rather than only for the back-dated ones.

It remains a single conversion, not a second opinion: fiat_amount_as_satoshis
is the same function create_invoice would have called, and the rate is
derived by the same formula calculate_fiat_amounts uses. Because passing
sats means LNbits no longer stamps the fiat metadata itself, payroll now
writes the identical fiat_currency / fiat_amount / fiat_rate / btc_rate keys
onto the payment, so a payroll payment still reads like any other
fiat-priced one in the payments list.

A rate that rounds an amount to zero sats now fails the period rather than
raising a zero-division while deriving the rate.

Memo omits the rate clause for sat-denominated contracts, where there is no
conversion to report.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jy52j9GRZ6XKa1Zt21LLj

tests/conftest.py

@@ -24,6 +24,7 @@ def make_contract(
     created_at: str = "2026-01-01",
     pricing_mode: PricingMode = PricingMode.payday,
     manual_rate: float | None = None,
+    label: str = "",
 ) -> Contract:
     return Contract(
         id=contract_id,
@@ -41,6 +42,7 @@ def make_contract(
         backfill=backfill,
         pricing_mode=pricing_mode,
         manual_rate=manual_rate,
+        label=label,
         created_at=datetime.strptime(created_at, "%Y-%m-%d").replace(
             tzinfo=timezone.utc
         ),