Add Cashier-equivalent Stripe billing module (subscriptions, invoices, webhooks)

[antosubash]

Background

There is no first-class billing/subscription module. SaaS apps building on SimpleModule have to integrate Stripe themselves: customer + subscription objects, webhooks, proration, trial flow, invoice PDFs, tax. Laravel Cashier solves all of this.

Motivation

• Multi-tenant SaaS is an obvious target audience for SimpleModule
• Billing is high-risk and high-effort to build well; commoditizing it is a big sell
• Webhook handling, proration, retries, and dunning are common foot-guns

Design sketch

New optional module modules/Billing (Stripe driver only at first; abstraction allows Paddle later):

Capabilities:

• IBilling service: CreateCustomerAsync, SubscribeAsync(customer, plan), SwapPlanAsync, CancelAsync, ResumeAsync, OnTrial(customer)
• IsSubscribed(customer) / Subscribed(plan) predicates
• BillingPortalAsync(returnUrl) — Stripe Customer Portal redirect
• CheckoutAsync(plan, successUrl, cancelUrl) — Stripe Checkout
• Webhooks endpoint at /billing/webhook with signature verification; emits IEventBus events (SubscriptionCreated, SubscriptionCanceled, InvoicePaid, InvoiceFailed)
• Invoices: Invoices(customer) returns paginated list; DownloadInvoicePdf(id)
• Models stored locally (mirrored from Stripe): Customer, Subscription, SubscriptionItem, Invoice
• Multi-tenant aware (Customer belongs to a tenant)

Permissions: Billing.View, Billing.Manage.

UI: admin pages — current subscription, plan switcher, payment methods, invoices.

Acceptance criteria

• Module scaffolded; Stripe driver via Stripe.net
• Webhook signature verification + idempotency (de-dupe by event_id)
• Trial + grace-period support
• Plan swap with proration
• Invoice PDF download (Stripe-hosted URL)
• Inertia admin UI
• xUnit tests with Stripe.NET mock + integration test against Stripe test mode
• Docs page with end-to-end setup walkthrough

References

• Laravel Cashier: https://laravel.com/docs/billing
• Stripe.net: https://github.com/stripe/stripe-dotnet