Bug: list_transactions returns empty description when invoice has memo instead

[noobedo]

Problem

The list_transactions NWC response returns empty description for payments that store the text in memo instead (common for keysend and LNURL-pay transactions).

Current code

In tasks.py:

"description": invoice_data.description,

Fix

"description": invoice_data.description or p.memo or "",

Impact

Wallet history in NWC apps shows blank descriptions for many transaction types.
Tested

Fixed on my LNbits instance — works correctly.