refactor: implement stealth api as rust crate

[satsfy]

Depends on #16 (it will be in draft mode until its merged)

Solves #10. This PR is a combination of rust refactors from @brenorb in #15 and me.

Implements an HTTP API for stealth as rust create stealth-api with POST /api/wallet/scan, request validation, structured error responses, RPC autodetection, and regtest end-to-end tests.

Replaces instructions on README on how to use stealth api.

Reviewer Notes

Open the branch locally, run cargo build to verify it compiles and run cargo test. Inspect the e2e test in api/tests/.

Run the api with cargo run --bin stealth-api and test that call:

curl 'http://localhost:20899/api/wallet/scan' \
  -H 'content-type: application/json' \
  -d '{"descriptor":"wpkh(xpub.../0/*)"}' | jq