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/routes/api/stripe/webhook.ts

@@ -0,0 +1,27 @@
+import type { APIEvent } from "@solidjs/start/server";
+import { stripe } from "~/server/stripe";
+import { handleWebhookEvent } from "~/server/services/billing.service";
+
+export async function POST(event: APIEvent) {
+  const body = await event.request.text();
+  const signature = event.request.headers.get("stripe-signature");
+
+  if (!signature) {
+    return new Response("Missing stripe-signature header", { status: 400 });
+  }
+
+  try {
+    const webhookEvent = stripe.webhooks.constructEvent(
+      body,
+      signature,
+      process.env.STRIPE_WEBHOOK_SECRET ?? "",
+    );
+
+    await handleWebhookEvent(webhookEvent);
+
+    return new Response("OK", { status: 200 });
+  } catch (err) {
+    const message = err instanceof Error ? err.message : "Webhook error";
+    return new Response(message, { status: 400 });
+  }
+}