RDD for passwordless auth

[ericelliott]

TL;DR:

• Passkey is first-class - primary sign-in mechanism.

Sign Up flow:

• Given a new user signing up, should prompt for email and auth with email magic link so that we have a reliable fallback to add/manage passkeys
• Given a user signing in with email magic link, after the magic link auth succeeds, should present the user with passkey auth
• Given a user attempting to sign in with the same magic link a second time, should inform the user that the link is expired and offer passkey auth, or a new link, instead

Sign in flow:

• Given an existing user signing in, should prompt for passkey authentication, and include a link to trigger a magic link flow in case they don't have a passkey on the current device.
• Given the user has successfully authenticated with magic link, should direct user to the course content landing page