fix: set a maximium sleep time when retrying to connect to the funding source (#2622)

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Co-authored-by: Pavol Rusnak <pavol@rusnak.io>

lnbits/app.py

@@ -206,7 +206,7 @@ async def check_funding_source() -> None:
             break
 
         retry_counter += 1
-        sleep_time = 0.25 * (2**retry_counter)
+        sleep_time = min(0.25 * (2**retry_counter), 60)
         logger.warning(
             f"Retrying connection to backend in {sleep_time} seconds... "
             f"({retry_counter}/{max_retries})"