Publish an edition

The money path (issue #61, split 7): reserves the requested items and creates the Payssage payment intent in one request. Prices are always server-computed from the DB (never trusted from the client). `purchase_items` is the availability ledger: the item rows are locked `FOR UPDATE` inside the checkout transaction before availability is checked, so two concurrent checkouts cannot oversell the last unit (the loser gets 409 listing the unavailable items). Flow: one transaction materializes the pending purchase (`purchases` + `purchase_items` + `registrations` / `product_purchases` / `program_participations`) and schedules the expiry job (`purchases.expire`, +10:01); the Payssage intent is created after commit; the intent id is stored back on the purchase in a second transaction. Only the Payssage webhook confirms payment (D3) — checkout never self-approves. Cards charge synchronously: the intent comes back `succeeded` but the purchase stays `pending` until the webhook confirms. Pix returns the QR (`qr_code` / `qr_code_base64`) in the response. Free orders (`total_cents == 0`) confirm immediately — no intent, no expiry job: the purchase is `approved` and materialized rows are created confirmed (badge emitted). Gifting: ticket lines are one-per-person (`quantity: 1`, one `attendee` each) — self-purchase sends the purchaser's own user_id/email/name; a gifted ticket sends the recipient's, resolved by the front from their email via IdentityX. `attendee.user_id` is trusted as-is. Buying N tickets for the same ticket type sends N lines. One active ticket per person per edition is enforced — the same attendee cannot appear twice in the cart (400), and any attendee who already holds a pending/confirmed registration for the edition is rejected (409); cancelled/expired registrations free the slot. Program lines require ≥1 ticket line in the same cart (a participation attaches to the ticket's registration); program quantity is fixed at 1. One active `pending` purchase per user+edition is enforced by a partial unique index — a second checkout returns 409 with the existing purchase's id (client-key idempotency is a follow-up).