Add Satoshi API as a fee estimate fallback

[Bortlesboat]

Summary

Adds Satoshi API, the Bitcoin fee API at bitcoinsapi.com, as one extra fallback source in the wallet fee estimator.

The existing order still tries mempool.space first. If that direct recommended-fees call fails, this PR tries Satoshi API's mempool-compatible free endpoint before falling back to the mempool-blocks heuristic and then Blockstream.

Why

The endpoint returns the same fee bucket shape the service already consumes:

{ fastestFee: 2.13, halfHourFee: 2.13, hourFee: 2.01, economyFee: 1.0, minimumFee: 1.0 }

That gives the wallet another no-auth fee source without changing defaults or transaction behavior.

Checks

• npx eslint services/fee-service.ts --max-warnings=0
• npx tsc --noEmit
• git diff --check
• curl -sS https://bitcoinsapi.com/api/v1/compat/mempool/fees/recommended

Note: full npm run lint -- --max-warnings=0 still reports an existing warning in app/wallet-setup.tsx (Array<T> style). I left that unrelated file untouched.

[jamespepper81]

Thanks for this - I’m just looking over some of your other work as well.

I’ll be open sourcing https://app.bitsleuth.ai soon, which is a Bitcoin wallet analysis tool that you might find interesting.

Feel free to check out the site as well: https://www.bitsleuth.ai/