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

docker-compose.yml

@@ -31,6 +31,12 @@ services:
       options:
         max-size: "10m"
         max-file: "3"
+    healthcheck:
+      test: ["CMD", "node", "-e", "fetch('http://localhost:3100/health').then(r=>{if(!r.ok)process.exit(1)}).catch(()=>process.exit(1))"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+      start_period: 30s
     mem_limit: 2560m
     cpus: 1.5