Adds the package's missing subscription lifecycle so any host app gets
duration-aware, product-linked subscriptions without re-implementing billing:
- Models: Subscription (extends Laravel\Cashier\Subscription) + SubscriptionItem,
in the package's UUID convention, resolved through shop.models/shop.tables.
Subscription gains product()/resolveProduct(), callProductActions() (runs the
product's ProductActions with the subscription + an access-expiry override),
and recordStarted()/recordRenewed()/recordCanceled() lifecycle hooks.
- Events: SubscriptionStarted / SubscriptionRenewed / SubscriptionCanceled,
carrying the Cashier subscription so host subclasses work too.
- Migration: UUID subscriptions / subscription_items tables (hasTable-guarded,
config table names, nullable product_id + current_period_* columns).
- ShopServiceProvider points Cashier at these models by default; opt out via
shop.subscriptions.register_cashier_models for apps that subclass Cashier.
Additive and backward-compatible (registration is config-gated, tables are
guarded). Adds SubscriptionLifecycleTest; full suite 1409 green. Docs + README.
