Implement Signup 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 signup information for a new user.

Additionally:

• ensure usernames are unique across the app
• allow a flexible method for enforce password constraints (e.g. for now, > 8 characters in length)

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

Acceptance criteria

• refactor / complete implementation of the auth/signup route so that signup with username+password succeeds
• non-unique usernames fail with appropriate message
• password constraint checks are triggered with appropriate message
• tests added