[DevOps] Add /health endpoint and Docker HEALTHCHECK

- src/routes/health/+server.ts: lightweight health endpoint returning 200 OK
- Dockerfile: HEALTHCHECK hitting /health every 30s, 5s timeout, 10s start period
  Enables Docker/Nginx zero-downtime routing and auto-recovery.

Dockerfile

@@ -43,4 +43,8 @@ COPY --from=builder /app/.env.production .env
 # SvelteKit (via adapter-node) defaults to port 3000.
 EXPOSE 3000
 
+# Healthcheck to verify the app is running
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+  CMD node -e "fetch('http://localhost:3000/health').then(r => r.ok ? process.exit(0) : process.exit(1)).catch(() => process.exit(1))"
+
 CMD ["node", "index.js"]

src/routes/health/+server.ts

@@ -0,0 +1,19 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+
+/**
+ * Health Check Endpoint
+ * Used by Docker and Nginx to verify the service is running.
+ */
+export const GET: RequestHandler = async () => {
+    return json({
+        status: 'healthy',
+        timestamp: new Date().toISOString(),
+        service: 'aether-app-sveltekit',
+        node_env: process.env.NODE_ENV || 'development'
+    }, {
+        headers: {
+            'Cache-Control': 'no-cache'
+        }
+    });
+};