The binary was hardcoded to talk to https://boltz.exchange/api via no
proxy, which made it useless for verifying the signet Boltz backend
added by the recent boltz_instances cherry-pick.
Read two new optional environment variables on startup:
BOLTZ_API_BASE URL prefix (default: https://boltz.exchange/api)
BOLTZ_PROXY libcurl proxy (e.g. socks5h://127.0.0.1:9050)
and pass them through to the Boltz::Connection constructor. No
behavior change when neither variable is set, so existing usage
continues to hit mainnet Boltz.
Tested against signet Boltz via both paths:
./dev-boltz-api /getpairs # mainnet, default
BOLTZ_API_BASE=http://127.0.0.1:8080 \
./dev-boltz-api /getpairs # signet via socat
BOLTZ_API_BASE=http://boltz7ck...onion \
BOLTZ_PROXY=socks5h://127.0.0.1:9050 \
./dev-boltz-api /getpairs # signet direct via Tor
