feat: k6 load testing web UI with Docker

- Multi-stage Dockerfile builds k6 from source (grafana/k6)
- Express backend with SSE live output streaming
- SQLite-backed test history with delete support
- Frontend: URL input, VUs, duration, RPS limit, custom headers
- Persisted via Docker volume, listens on port 8118

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/package.json

@@ -0,0 +1,14 @@
+{
+  "name": "k6-web-ui-backend",
+  "version": "1.0.0",
+  "description": "Web UI backend for k6 load testing",
+  "main": "server.js",
+  "scripts": {
+    "start": "node server.js"
+  },
+  "dependencies": {
+    "express": "^4.18.2",
+    "better-sqlite3": "^9.4.3",
+    "uuid": "^9.0.0"
+  }
+}