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>

Dockerfile

@@ -0,0 +1,35 @@
+# Build k6 from source
+FROM golang:1.22-alpine AS k6-builder
+
+RUN apk add --no-cache git
+
+WORKDIR /src
+RUN git clone --depth=1 https://github.com/grafana/k6.git .
+RUN go build -o /usr/local/bin/k6 .
+
+# Final image
+FROM node:20-alpine
+
+# Copy k6 binary from builder
+COPY --from=k6-builder /usr/local/bin/k6 /usr/local/bin/k6
+
+# Install backend dependencies
+WORKDIR /app/backend
+COPY backend/package.json .
+RUN npm install --production
+
+COPY backend/server.js .
+
+# Copy frontend
+COPY frontend/ /app/frontend/
+
+# Data volume for SQLite history
+RUN mkdir -p /data
+VOLUME /data
+
+ENV PORT=8118
+ENV DB_PATH=/data/history.db
+
+EXPOSE 8118
+
+CMD ["node", "/app/backend/server.js"]