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>

docker-compose.yml

@@ -0,0 +1,11 @@
+services:
+  k6-ui:
+    build: .
+    ports:
+      - "8118:8118"
+    volumes:
+      - k6-data:/data
+    restart: unless-stopped
+
+volumes:
+  k6-data: