As a new user, I want to sign in with Facebook, so that I don't have to create another password

Acceptance criteria

• Given I am on the sign-in screen, when I select "Continue with Facebook", then I am taken to Facebook's authorization page
• Given I authorize the app, when I return, then I am signed in and my name and email are populated
• Given I decline authorization, when I return, then I see the sign-in screen with a clear message and no error state
• Given an account already exists with the same email, when I sign in with Facebook, then the accounts are linked rather than duplicated

Out of scope

• Requesting any Facebook permission beyond name and email
• Posting to Facebook on the user's behalf

Open questions

• Facebook's app review process can take weeks. Do we have a developer account and is it approved? This is why the story is tagged Needs Refinement.
• Is Facebook sign-in still worth it given declining usage in our segment? Worth checking before we spend the points

Definition of Done

• Acceptance criteria pass
• Account linking tested against an existing email
• Code reviewed and merged
• Tests written and green
• Accepted by Product Owner