Checkout completed

Sent when the DePix settles to the merchant wallet (terminal success). On the depix rail it means the payer's transaction reached two confirmations. Sent to the checkout's callback_url (checkout-level, falling back to product/merchant defaults at creation time). Outbound POST to the merchant's callback URL, signed with X-DePix-Signature: t=<unix ts>,v1=<hex hmac-sha256> where the HMAC is computed over "${t}.${raw body}" with the merchant webhook secret. Verify the signature and reject stale timestamps. Delivery is retried on failure: immediate → 1m → 10m → 1h → 4h → 12h (max 6 attempts); use data.event_id to deduplicate. Respond with any 2xx quickly to acknowledge.