Implement Login Route

[lululeon]

For now, we will handle basic signup with username and password only. This ticket will leverage the lucia auth middleware to receive and process login information for a new user.

Refs: https://v2.lucia-auth.com/guidebook/sign-in-with-username-and-password/

Acceptance criteria

• refactor / complete implementation of the /auth/signin route so that login with username+password succeeds
• successful login yields a valid token (for use by the caller)
• tests added