list_transactions: fall back to extra.comment / memo when BOLT11 description is empty

[bitcoin3us]

Closes #42.

Summary

_on_list_transactions returns description: None for LNURL-pay payments because the BOLT11 description is a SHA-256 hash (LUD-06 description_hash), not human text. The actual content lives in:

• payment.extra.comment — the payer's LUD-12 message
• payment.memo — the Pay Link's "Item description"

…both of which list_transactions was ignoring. lookup_invoice already falls back to payment.memo; this PR mirrors that, plus prefers LUD-12 comments when present.

Change

One field's value, in tasks.py _on_list_transactions:

"description": (
    (p.extra or {}).get("comment")     # LUD-12 payer-supplied message
    or invoice_data.description        # BOLT11 description (when human text, not hash)
    or p.memo                          # Pay Link item description / generic memo
),

Ordering rationale — when a payer typed a message via LUD-12, that's more informative for the receiver than the receiver's own Pay Link template. Falls back through both other sources for non-LUD-12 incoming payments and for outgoing ones, matching the existing _on_lookup_invoice shape.

Test plan

• Read against the divergent _on_lookup_invoice body (line ~356) — fallback chain is consistent with what lookup already does, plus the extra.comment source.
• Local LNBits dev env: install the change, send a comment-bearing LUD-12 payment to a Lightning Address, confirm description in list_transactions response equals the payer's comment string.
• Same flow with no comment: confirm fallback to invoice_data.description (if present) or p.memo.
• Outgoing payment (pay_invoice): confirm description still populates from invoice_data.description / p.memo.

Happy to run any additional tests the maintainer wants. Verified on Lightning Piggy (ESP32 NWC display wallet, github.com/LightningPiggy/LightningPiggyApp) against an LNBits instance: with this change, payer-supplied comments now appear in the on-device transaction list; without it, every LNURL-pay row reads as bare-amount.

[riccardobl]

Looks good! Thanks!