Public Play Test Opt In

Capture a closed-testing opt-in from the public /play-testing form.

Idempotent per account email — a re-submit updates the same row (e.g. a corrected Play Store Gmail). No auth required; the form prefills from the session when signed in, but anyone with the email link can consent. The admin never reads a reply: consenting submissions land directly in v2.play_test_optins and surface on the admin console.