btc402: paid Bitcoin node data service

Built on Bitcoin Core (unmodified). This project is NOT a fork of Bitcoin Core.
It runs the official, unmodified Bitcoin Core snap build (31.1) as published by the
Bitcoin Core project (https://bitcoincore.org, https://bitcoin.org).
Bitcoin Core is Copyright (c) the Bitcoin Core developers and is distributed
under the MIT software license. Credit to the Bitcoin Core contributors.

What this project adds on top (our own code and configuration):
  - a pruned-node deployment (prune=10000) on Ubuntu under WSL2
  - btc_x402.py: x402 per-call payment layer over read-only node RPC
  - lightning.py: Lightning rail (API keys, BOLT11 top-ups, monthly plans in sats)
  - registry.py: the UnyKorn chain registry (offers + hashed payment-verification records)
  - btc_wallet.sh: helper for the node's receiving wallet