feat(cloud): Add Stripe subscriptions via Cashier mirror

[nfebe]

Task

Make the cloud plugin the local mirror of Stripe subscription state, so the app can read subscription status without calling Stripe on the request path.

Actions

• Require laravel/cashier in the cloud plugin composer (not the root app).
• Add a cloud-owned BillingCustomer model (Cashier Billable, one-to-one with user_id); point cashier.model at it so the core User model stays free of any billing trait or columns.
• Ship the Cashier migrations inside the plugin; register the Cashier webhook route; add a checkout endpoint for the existing plans.

Acceptance

• A Stripe test checkout creates/updates local subscription rows via the webhook.
• Subscription state resolves from local tables with no Stripe call on the request path.
• The core User model has no billing trait or columns.