Grant Studio Subscription Admin

Attach (or cancel) a COMP studio subscription — no Stripe (admin API key).

Body: {user_id? | email?, plan, billing_period?='monthly', months?} to attach, or {user_id?|email?, cancel: true} to cancel. A comp subscription sets the wallet's studio_* fields to an active subscription with studio_subscription_id = NULL (the marker for manual/non-Stripe), and grants the plan's credit allotment with a matching expiry — mirroring exactly what the invoice webhook does for a paid subscription, so the consumed amount and scale stay identical. Lets support comp a plan without a card.