feat(billing): add subscription and Stripe billing router

- Add stripeCustomerId column to users table
- Create Stripe client initialization (web/src/server/stripe.ts)
- Add billing service with getOrCreateCustomer, checkout/portal sessions,
  subscription management, invoice listing, and webhook event handling
- Create billing tRPC router with getSubscription, createCheckoutSession,
  createPortalSession, cancelSubscription, reactivateSubscription, listInvoices
- Add raw webhook endpoint at /api/stripe/webhook with signature verification
- Define Valibot schemas for all billing procedure inputs
- Wire billing router into root tRPC router
- Update schema tests for new column/index counts
- Write unit tests for billing service and router

web/src/server/api/schemas/billing.ts

@@ -0,0 +1,24 @@
+import { object, string, url, minLength, optional, picklist } from "valibot";
+
+export const CreateCheckoutSessionSchema = object({
+  priceId: string([minLength(1)]),
+  successUrl: string([url()]),
+  cancelUrl: string([url()]),
+});
+
+export const CreatePortalSessionSchema = object({
+  returnUrl: string([url()]),
+});
+
+export const CancelSubscriptionSchema = object({
+  subscriptionId: string([minLength(1)]),
+});
+
+export const ReactivateSubscriptionSchema = object({
+  subscriptionId: string([minLength(1)]),
+});
+
+export const ListInvoicesSchema = object({
+  limit: optional(string(), "10"),
+  startingAfter: optional(string()),
+});