security: pre-release hardening, automated testing, and production readiness

SECURITY (critical fixes):
- Add authMiddleware to /api/system/console-token (was publicly accessible)
- Validate api_url on node create/update to prevent SSRF (rejects localhost/loopback)
- Add rate limiting (5 req/15 min/IP) to /api/auth/login and /api/auth/setup
- Fix path traversal in env_file resolution — absolute/escaping paths rejected
- Add stack name validation to GET routes (was only on PUT/POST)
- Add helmet security headers middleware
- Restrict CORS to FRONTEND_URL in production

PRODUCTION READINESS:
- Add GET /api/health public endpoint + HEALTHCHECK in Dockerfile
- Add SIGTERM/SIGINT graceful shutdown handler (drains connections, closes DB)
- Run container as non-root sencho user in Dockerfile

QUALITY:
- Fix 4 silent empty catch{} blocks in EditorLayout (now show toast.error)
- Connect ErrorBoundary to root App in main.tsx
- Replace WebSocket.Server with named WebSocketServer import (ESM compat)

TESTING (new automated test suite):
- Install Vitest; 38 backend tests across 4 suites covering validation utilities,
  health endpoint, auth middleware, login flows, SSRF protection, and path traversal
- Extract isValidStackName/isValidRemoteUrl/isPathWithinBase to utils/validation.ts
- Playwright E2E scaffolding: auth, stacks, nodes specs + shared login helper
- CI: run Vitest + ESLint on every PR

Dockerfile

@@ -58,8 +58,21 @@ COPY --from=frontend-builder /app/frontend/dist ./public
 # Set environment to production
 ENV NODE_ENV=production
 
+# Create a non-root user and ensure the data/compose directories are writable.
+# The actual volume paths are mounted at runtime, so we only pre-create the
+# default data dir here; the compose dir is user-supplied via COMPOSE_DIR.
+RUN addgroup -S sencho && adduser -S -G sencho sencho \
+  && mkdir -p /app/data \
+  && chown -R sencho:sencho /app
+
+USER sencho
+
 # Expose port
 EXPOSE 3000
 
+# Health check — polls the public /api/health endpoint every 30s
+HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
+  CMD node -e "const h=require('http');h.get('http://localhost:3000/api/health',r=>{process.exit(r.statusCode===200?0:1)}).on('error',()=>process.exit(1))"
+
 # Start the server
 CMD ["node", "dist/index.js"]