Add dbt source

Non-sensitive view of a dbt source for API responses — never includes the decrypted connection (which holds credentials). The primary source is synthesised from the project's own dbt_connection. repository, branch and projectSubPath are the git-backed source's identity (null for non-git connections); they are safe to expose — only secrets are stripped.

hasCredentialError is always false for the synthesised primary source. See ProjectDbtSource for what it means on an additional source.