Migrate to Spring Boot + MyBatis and add REST APIs

[Isga1]

Motivation

• Modernize the project by replacing the legacy servlet + JDBC stack with a Spring Boot application scaffolded for REST services.
• Introduce MyBatis-based persistence to simplify SQL mapping and paging for staff management.
• Harden authentication by switching password handling to BCrypt and documenting DB schema and admin creation.
• Provide a clean API surface (auth + staff CRUD) suitable for modern clients.

Description

• Replace pom.xml to use Spring Boot, add application.yml, and include mybatis-spring-boot-starter and jbcrypt dependencies.
• Add application entry point UserManagerApplication and new REST controllers AuthController and StaffController with DTOs under src/main/java/com/example/dto.
• Implement service layer and MyBatis mappers (AdminMapper, StaffMapper) with mapper XMLs in src/main/resources/mappers and a paginated StaffServiceImpl implementation.
• Remove legacy servlet/JDBC helpers and replace PasswordUtil to use BCrypt, add src/main/resources/schema.sql, and update README.md with setup instructions.

Testing

• No automated tests were executed for this change.

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Codex Task

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