Create Guest Gift Checkout

Create a gift checkout session for a buyer WITHOUT a Pixley account.

The buyer supplies their email (pre-filled into Stripe checkout); the recipient gets the standard redeem email after payment. Fulfillment is the same webhook path as authed gifts — purchaser_user_id is simply empty.

Rate limited per real client IP (_client_ip_key reads the ALB-stamped hop, not the attacker-controlled XFF[0]) by the app-wide slowapi limiter.