fix(admin): migrate dashboard passwords to bcrypt

[honglei]

Summary

• store newly created and updated dashboard user passwords with bcrypt instead of direct SHA-512 hex
• preserve compatibility with existing SHA-512 records by upgrading them to bcrypt after the first successful login
• accept legacy SHA-512 old-password verification in the modify-password flow so upgrades are not breaking
• switch the seeded H2 admin password to a bcrypt hash

Validation

• shenyu-admin compiles successfully in the reactor build
• targeted tests passed: DashboardUserServiceTest and DashboardUserControllerTest

[Aias00]

pls fix ci

[Aias00]

The seed hash uses cost 12 ($2b$12$...), but the runtime encoder new BCryptPasswordEncoder() defaults to cost 10 ($2a$10$). This means the built-in admin account has strength 12 while all newly created/modified user passwords get strength 10.

Fix: Either use new BCryptPasswordEncoder(12) in PasswordHashService, or regenerate the seed hash at cost 10 for consistency.