add openapi generation

[mikedotexe]

Summary

This moves explorer-api to owning its aggregate OpenAPI document in-repo.

The checked-in openapi/openapi.yaml file is now generated from Rust request/response models plus a Rust-owned operation registry, instead of being maintained outside the service repo.

What changed

• add an openapi feature with a generate-openapi binary
• add src/openapi.rs to build the aggregate OpenAPI document
• check in openapi/openapi.yaml
• move shared app types, routes, and docs handlers into lib.rs so both the server binary and the OpenAPI generator can reuse them
• update README docs to describe the current docs pipeline

This covers the /v0 transactions, account, block, blocks, and receipt endpoints.

Why the main.rs -> lib.rs split

This is not intended as an architectural rewrite.

Rust binaries cannot be imported by other binaries, so the new generate-openapi binary needed a small shared library surface for:

• app state types
• route scopes and docs handlers
• request and response DTOs

The server startup flow remains in main.rs; the shared reusable pieces moved to lib.rs.

What did not change

• the public HTTP routes and handlers remain the same
• the larger diff is mostly the checked-in aggregate OpenAPI output and typed schema exposure, not new runtime behavior
• server startup, env loading, logging, and binding remain in main.rs
• portal-owned per-operation splitting and docs enhancements still live outside this repo

Validation

cargo check
cargo run --features openapi --bin generate-openapi -- --check