Activate Subscription

Activate a subscription. When from_workflow=true (Restate calls): - Returns ActivateSubscriptionResponse with status field - Returns "skipped" if trial still active (no error) - This prevents premature activation if trial was extended after scheduling When from_workflow=false (public API calls): - Returns SubscriptionResponse - Raises 400 error if trial still active Requires the initial invoice (billing_reason='subscription_create') to be paid. Works for both User and API Key authentication. Account access is automatically validated via endpoint dependency.