Create a Polar checkout session for PRO tier upgrade.
Required Role: ADMIN or higher
Creates a unique checkout session URL on Polar for the team to upgrade to PRO tier. The frontend should open this URL in a new tab/window.
Returns 409 Conflict if team already has an active subscription.
Note: The checkout URL is unique per request and can only be used once.
Bearer authentication header of the form Bearer <token>, where <token> is your auth token.