Create Checkout

Start a Polar checkout for the Pro plan and hand back its URL.

The session is bound to users.id as Polar's external customer id, which is what lets the resulting customer.state_changed webhook find this account again.

Reconciles first. If a webhook was lost, our row says "free" while Polar says "active" — and sending that user to checkout produces a dead end, because Polar refuses with "You already have an active subscription" and the user is left staring at a red error on a page they cannot complete. Pulling the real state first turns that into a 409 the client can act on.