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>

packages/email/src/providers/plunk/env.ts

@@ -0,0 +1,21 @@
+import { defineEnv } from "envin";
+import * as z from "zod";
+
+import { envConfig } from "@turbostarter/shared/constants";
+
+import { sharedPreset } from "../../utils/env";
+
+import type { Preset } from "envin/types";
+
+export const preset = {
+  id: "plunk",
+  server: {
+    PLUNK_API_KEY: z.string(),
+  },
+  extends: [sharedPreset],
+} as const satisfies Preset;
+
+export const env = defineEnv({
+  ...envConfig,
+  ...preset,
+});