Fix three critical issues: Docker healthcheck, USD->EUR pricing, static asset caching

- Docker healthcheck: Use Node.js instead of curl (not installed in slim image)
- Pricing: Change from USD ($) to EUR (€) in frontend and backend Stripe integration
- Static assets: Add Cache-Control headers (1 day) for /public and /docs files

src/routes/billing.ts

@@ -192,7 +192,7 @@ async function getOrCreateProPrice(): Promise<string> {
   const price = await getStripe().prices.create({
     product: productId,
     unit_amount: 900,
-    currency: "usd",
+    currency: "eur",
     recurring: { interval: "month" },
   });