fix(session): price the balance from the warm rate cache only

/session gates the terminal unlocking, and satoshis_amount_as_fiat() on a
cold cache queries external exchanges (~1 s measured on l484). Read the
LNbits btc-price cache directly instead: warm → fiat, miss → null and
the terminal prices the sats itself. No rate lookup ever blocks a tap.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

views_lnurl.py

@@ -8,7 +8,7 @@ from fastapi import APIRouter, HTTPException, Query, Request
 from lnbits.core.crud import get_wallet
 from lnbits.core.services import create_invoice, pay_invoice
 from lnbits.settings import settings
-from lnbits.utils.exchange_rates import satoshis_amount_as_fiat
+from lnbits.utils.cache import cache
 from lnurl import (
     CallbackUrl,
     LightningInvoice,
@@ -507,8 +507,10 @@ async def api_session(p, c, request: Request, external_id: str):
 
     # Balance + fiat, the way the LNbits wallet page shows them: the wallet's
     # own currency first (per-wallet setting, LNbits ≥ 1.6), then the
-    # instance's default accounting currency; no currency → no fiat, and a
-    # rate failure never fails the session.
+    # instance's default accounting currency. The fiat figure comes ONLY from
+    # LNbits' already-warm rate cache: this response gates the terminal
+    # unlocking, and a cold lookup queries external exchanges (~1 s). Cache
+    # miss → `fiat: null`; the terminal prices the sats itself.
     wallet = await get_wallet(card.wallet)
     balance_msat = int(wallet.balance_msat) if wallet else 0
     currency = (getattr(wallet, "currency", None) if wallet else None) or getattr(
@@ -516,10 +518,9 @@ async def api_session(p, c, request: Request, external_id: str):
     )
     fiat = None
     if currency:
-        try:
-            fiat = await satoshis_amount_as_fiat(balance_msat / 1000, currency)
-        except Exception:
-            fiat = None
+        price = cache.get(f"btc-price-{currency}")  # fiat per BTC, as btc_price()
+        if isinstance(price, (int, float)) and price > 0:
+            fiat = balance_msat / 1000 / 100_000_000 * float(price)
 
     return {
         "authenticated": True,