Subscription tier removed; everything is credits

The old monthly/yearly subscription tier, paywall, seat management, and "subscribers-only" app flag have been retired. Billing is now uniformly credits-based: buy a pack, optionally enable auto-refill, and every app and integration draws from the same balance. The Subscription tab, subscription card, paywall component, requireSubscription SDK call, and Subscriber/Free labels in the nav are all gone.