feat: Add email change via Stytch magic link

[paulvanbrenk]

Summary

• Users can now change their email from the Settings page via Stytch magic link verification
• New dedicated /confirm-email page handles the magic link callback, authenticates the token, syncs the new email, and removes the old email from Stytch
• Backend POST /api/users/me/confirm-email-change endpoint cleans up old emails from Stytch and updates the DB
• Stytch client extended with DeleteEmailAsync and full emails array on GetUserAsync

Test plan

• Enter a new email on Settings, click Send Verification — check email arrives
• Click magic link — redirected to /confirm-email — see success message
• Verify Settings page now shows the new email
• Verify old email is removed from Stytch (only new email remains)
• Verify GET /api/users/me returns the new email
• Try logging in with the new email — should work
• Try logging in with the old email — should fail (removed from Stytch)

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