[Multi-Tenant] Remove Kratos requirement

[njlie]

Using http-signatures as an identity management method is sufficient for both UI-based and programmatic API calls, so it renders Kratos redundant.

• Remove kratosIdentityId from tenants table
• Remove startup check to add a tenant to database & check if it exists in Kratos
• Remove check on Admin UI for Kratos session
• Remove Kratos call from tenant creation logic

[njlie]

Closing as we are starting fresh from a new feature branch and Kratos is not mandatory in main.