chore: dockerize rust backend, add auth/cancel tests, generate openap…

[kingsebuks001]

…i spec

• Add multi-stage Dockerfile for backend service
• Wire backend service into docker-compose with healthcheck to /health/rpc
• Extend cancel.rs tests with authorized/unauthorized merchant cases
• Add auth.rs unit tests for expired challenges and unique nonces
• Annotate routes with utoipa and serve OpenAPI spec at /openapi.json
• Add test asserting /openapi.json returns 200 and valid JSON

Pull Request

Summary

• Describe the purpose of this PR in one or two sentences.

Checklist

• Closes #__
• Tests added or updated
• ABI snapshot updated if contract sources changed (abis/ and COMEBACKHERE-contracts/)
• Screenshots attached if UI changed
• Documentation updated if needed

Notes

• For contract changes, verify ABI snapshot hygiene with make check-abi-snapshots.
• For UI changes, include screenshots or screen recordings to help reviewers.
• closes
• closes Add auth.rs unit test coverage #201
• closes Add cancel_invoice authorization test #202
• closes Dockerize the Rust backend and add a healthcheck to docker-compose #203
• closes Generate an OpenAPI spec for the Rust backend #204

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