feat: Add Admin user role and category authorization

[gilito11]

Summary

• Implement Admin user entity with ROLE_ADMIN authority
• Add AdminRepository and AdminEventHandler for admin user management
• Create CategoryEventHandler to enforce admin-only access for category creation
• Update DBInitialization to create default admin user (username: admin, password: password)
• Update Category feature tests to use admin authentication
• Re-enable authorization test to verify non-admin users cannot create categories

Changes

• New files:

  • src/main/java/cat/udl/eps/softarch/demo/domain/Admin.java - Admin entity extending User
  • src/main/java/cat/udl/eps/softarch/demo/repository/AdminRepository.java - Admin repository
  • src/main/java/cat/udl/eps/softarch/demo/handler/AdminEventHandler.java - Password encoding handler
  • src/main/java/cat/udl/eps/softarch/demo/handler/CategoryEventHandler.java - Authorization enforcement

• Modified files:

  • src/main/java/cat/udl/eps/softarch/demo/config/DBInitialization.java - Added admin user initialization
  • src/test/resources/features/Category.feature - Updated scenarios to use admin login

Test plan

• Category creation scenarios use admin authentication
• Non-admin users receive 403 when attempting to create categories
• Admin user is created on application startup
• All validation scenarios still work correctly

[francesc-contreras-tech]

Hello @gilito11, please check the reason why CI/CD have failed.