[codex] Improve Spring Boot MySQL application structure

[DoRmAmMu1997]

Summary

• Refactors the REST resources to constructor-injected service classes while preserving the existing /rest/... route shape.
• Adds shared API error handling, safer missing-record and duplicate-record behavior, password write-only JSON handling, and environment-driven MySQL config.
• Adds H2-backed Spring tests, Java 11 GitHub Actions CI, .gitignore, Maven wrapper metadata, and a project-specific README explaining structure, setup, routes, and tests.

Test Plan

• pom.xml parsed successfully as XML locally.
• git diff --check completed with no whitespace errors.
• Static scan found no remaining production @Autowired, .findById(...).get(), broad catch(Exception), or removed custom validator references.
• ./mvnw.cmd test could not run locally because this environment has no Java/JAVA_HOME configured.
• GitHub Actions Java CI passed on Java 11 for commit ee9290e139bb3d3dab6de740cb4ecb73c6f2b6fa: https://github.com/DoRmAmMu1997/My-SpringBoot-MySQL-Application/actions/runs/25627390275

Notes

• Existing public endpoint paths are intentionally preserved.
• Password hashing is out of scope for this educational cleanup pass; password fields are now hidden from JSON responses.