feat: modernize and secure Palette Core API

[srknskr]

Summary

• upgrade the API from .NET Core 3.1 to .NET 10 and current EF Core packages
• require JWT authentication by default and protect user/API-key administration with an admin policy
• replace public entity binding with validated DTO contracts
• hash passwords and stop exposing password/API-key values in read responses
• add an initial EF Core migration, health check, Swagger bearer support, integration tests, and CI
• update the README to match the modernized implementation

Verification

• Release build: 0 warnings, 0 errors
• Tests: 5 passed, 0 failed