[Quality] Bind gateway bootstrap API and public documentation

[seonghobae]

Quality / acquisition gap

Protected develop@622e5e6c3d534f230c390f10e3832efadfc01825 contains an independently runnable ZuulGatewayApplication bootstrap class with @SpringBootApplication and @EnableDiscoveryClient, but the public class and main(String[]) entry point have no beginner-readable Javadoc. The gateway module has behavioral security tests in active PR #142, but protected source has no focused bootstrap contract analogous to the existing bounded Eureka/Config Server bootstrap work.

This issue is intentionally path-disjoint from #142. It must not alter gateway authentication, routing, application.yml, pom.xml, identity-provider configuration, or security dependencies.

RCA

• Immediate cause: the public bootstrap class and method are undocumented.
• Technical cause: module startup/annotation identity has no focused contract test, so refactors can remove discovery/bootstrap annotations without a local structural failure and public API documentation requirements are not enforced for this entry point.
• Control gap: repository-wide coverage/docstring completeness issue [Quality evidence] Extend non-vacuous 100% coverage to every owned production module #205 is broader and still pending; this slice establishes a real gateway bootstrap boundary without claiming full gateway-module coverage.

TDD / acceptance

1. Add a focused test first that preserves @SpringBootApplication, @EnableDiscoveryClient, and a public-static main(String[]) entry point.
2. The same fail-first test requires beginner-readable class/method Javadoc explaining that the class starts the standalone mightyETL gateway, that discovery-client startup is part of bootstrap, and what args represents.
3. Observe hosted RED only at the missing documentation assertion; setup/import/fixture/dependency failure does not qualify.
4. Apply the smallest GREEN by adding Javadocs only; do not change annotations or SpringApplication.run(...) behavior.
5. Run full applicable CI/dependency/SBOM/SAST/security evidence on the exact candidate.
6. Keep active PR status separate from shipped truth and keep repository-wide coverage [Quality evidence] Extend non-vacuous 100% coverage to every owned production module #205, gateway JWT fix(gateway): replace placeholder token handling with JWT resource server #142, exact-source controls, scanner completeness and protected merge governance as independent acceptance dimensions.