feat: turbostarter boilerplate

Production-ready Next.js boilerplate with:
- Runtime env validation (fail-fast on missing vars)
- Feature-gated config (S3, Stripe, email, OAuth)
- Docker + Coolify deployment pipeline
- PostgreSQL + pgvector, MinIO S3, Better Auth
- TypeScript strict mode (no ignoreBuildErrors)
- i18n (en/es), AI modules, billing, monitoring

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.context/turbostarter-framework-context/sections/web/billing/webhooks.md

@@ -0,0 +1,41 @@
+---
+title: Webhooks
+description: Handle webhooks from your billing provider.
+url: /docs/web/billing/webhooks
+---
+
+# Webhooks
+
+TurboStarter handles billing webhooks to update customer data based on events received from the billing provider.
+
+Occasionally, you may need to set up additional webhooks or perform custom actions with webhooks.
+
+In such cases, you can customize the billing webhook handler in the billing router at `packages/api/src/modules/billing/router.ts`.
+
+By default, the webhook handler is configured to be as straightforward as possible:
+
+```ts title="router.ts"
+import { webhookHandler } from "@turbostarter/billing/server";
+
+export const billingRouter = new Hono().post("/webhook", (c) =>
+  webhookHandler(c.req.raw),
+);
+```
+
+However, you can extend it using the callbacks provided from `@turbostarter/billing` package:
+
+```ts title="router.ts"
+import { webhookHandler } from "@turbostarter/billing/server";
+
+export const billingRouter = new Hono().post("/webhook", (c) =>
+  webhookHandler(c.req.raw, {
+    onCheckoutSessionCompleted: (sessionId) => {},
+    onSubscriptionCreated: (subscriptionId) => {},
+    onSubscriptionUpdated: (subscriptionId) => {},
+    onSubscriptionDeleted: (subscriptionId) => {},
+    onEvent: (rawEvent) => {},
+  }),
+);
+```
+
+You can provide one or more of the callbacks to handle the events you are interested in.