* feat: add lnbits payment processor
* fix: add lnbits error logging and add lnbits config
* feat: use HMAC instead of IP whitelist for LNbits
also adds two utility functions and ensures the SECRET environment
variable is set.
* refactor: remove unnecessary comment
* fix(pay-to-relay/lnbits): compare by msat scaled amount
* fix: scale balance addition with invoice unit on confirm_invoice