Refactor: Migrate JPA to R2DBC and Hibernate Reactive

[yujuncai]

This commit migrates the data access layer from Spring Data JPA to Spring Data R2DBC and Hibernate Reactive.

Changes include:

• Updated pom.xml to include R2DBC, r2dbc-mysql, and Hibernate Reactive dependencies and remove JPA dependencies.
• Reconfigured application.yaml for R2DBC connection.
• User entity remains largely the same, compatible with Hibernate Reactive.
• UserRepository refactored to extend ReactiveCrudRepository, with methods returning Mono/Flux.
• SqlController and AuthController updated to use the reactive UserRepository, removing blocking calls and Schedulers.boundedElastic().
• SecurityAppApplication updated with @EnableR2dbcRepositories.

Testing:

• SecurityAppApplicationTests (context load) is PASSING.
• SqlControllerTest (with mocked UserRepository) is PASSING.
• UserRepositoryTest encountered persistent context configuration issues and is NOT reliably passing. Further work is needed for these specific repository tests.