feat: add rate endpoint per currency (#2641)

this is already used and implemented by tpos, lnurlp and i probably need
it for satspay aswell

lnbits/core/views/api.py

@@ -33,6 +33,7 @@ from lnbits.settings import settings
 from lnbits.utils.exchange_rates import (
     allowed_currencies,
     fiat_amount_as_satoshis,
+    get_fiat_rate_satoshis,
     satoshis_amount_as_fiat,
 )
 
@@ -200,6 +201,12 @@ async def api_perform_lnurlauth(
     return ""
 
 
+@api_router.get("/api/v1/rate/{currency}")
+async def api_check_fiat_rate(currency: str) -> Dict[str, float]:
+    rate = await get_fiat_rate_satoshis(currency)
+    return {"rate": rate}
+
+
 @api_router.get("/api/v1/currencies")
 async def api_list_currencies_available() -> List[str]:
     return allowed_currencies()