StripeTierProvisioner

Auto-creates and maintains Stripe Products and Prices for built-in membership tiers. Ships in feature/membership-signup-checkout. Hook-driven — registers on benecaster_membership_tier_created, benecaster_membership_tier_updated, benecaster_membership_tier_deleted, and benecaster_stripe_keyset_configured. Stripe Prices are immutable — when a tier price changes a new Price is minted and existing subscribers are grandfathered. Free and inactive tiers are skipped on all triggers.