Activate Subscription

Turn a paid Checkout Session for the Unlimited plan into a seeded workspace.

The session is re-read from Stripe rather than trusted from the client, the same way /activate treats an intent. Only a paid subscription session for the Unlimited plan is accepted. The webhook activates the plan on the user row on its own schedule; this endpoint writes the same facts first so the workspace is never opened as a credit account in the seconds before the webhook lands (the sender caches that answer for five minutes).