As an iOS user, I want to sign in with Apple, so that I can use the app without sharing my email

Why this one is not optional

Apple's App Store guidelines require Sign in with Apple on any app that offers third-party social sign-in. Ship Google or Facebook without this and the release gets rejected.

Acceptance criteria

• Given I am on the sign-in screen in the iOS app, when I select "Sign in with Apple", then the native Apple sheet appears
• Given I choose "Hide My Email", when I complete sign-in, then the account is created against Apple's relay address and email delivery still works
• Given I have signed in before, when I sign in again, then I reach the same account, not a new one
• Given I revoke access in iOS Settings, when I next open the app, then I am signed out cleanly rather than stuck

Out of scope

• Sign in with Apple on web and Android (separate stories if we want them)

Definition of Done

• Acceptance criteria pass
• Relay email delivery verified end to end
• Revocation path tested
• Code reviewed and merged
• Tests written and green
• Accepted by Product Owner