feat: add dev Dockerfile with VNC/X11/Xvfb and update docker-compose.dev.yml

- Create deploy/docker/backend-dev.Dockerfile: single-stage, root,
  includes xauth, x11vnc, novnc, websockify, xvfb, Playwright+chromium,
  Python3+pip+ocr-worker (--break-system-packages)
- Use HTTP (not HTTPS) for apt mirrors per BuildKit TLS note
- Update docker-compose.dev.yml backend to point to backend-dev.Dockerfile
- .dockerignore unchanged (no issues found)

deploy/docker/backend-dev.Dockerfile

@@ -0,0 +1,47 @@
+# Dev Dockerfile — single-stage, root, VNC/X11 + Playwright + OCR
+# For production, see backend.Dockerfile (multi-stage build)
+FROM node:22-bookworm
+
+ARG DEBIAN_MIRROR=mirrors.cloud.tencent.com
+ARG NPM_REGISTRY=https://registry.npmmirror.com
+ARG UV_INDEX_URL=https://mirrors.cloud.tencent.com/pypi/simple
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV TZ=Asia/Shanghai
+ENV PLAYWRIGHT_BROWSERS_PATH=/ms-playwright
+ENV PIP_INDEX_URL=${UV_INDEX_URL}
+ENV NPM_CONFIG_REGISTRY=${NPM_REGISTRY}
+
+WORKDIR /app
+
+RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    sed -i "s|http://deb.debian.org/debian|http://${DEBIAN_MIRROR}/debian|g" /etc/apt/sources.list.d/debian.sources \
+  && sed -i "s|http://security.debian.org/debian-security|http://${DEBIAN_MIRROR}/debian-security|g" /etc/apt/sources.list.d/debian.sources \
+  && apt-get update \
+  && apt-get install -y --no-install-recommends \
+       curl ca-certificates python3 python3-pip tzdata \
+       xauth x11vnc novnc websockify xvfb \
+  && ln -snf /usr/share/zoneinfo/${TZ} /etc/localtime \
+  && echo ${TZ} > /etc/timezone
+
+COPY apps/backend/package.json apps/backend/package-lock.json ./
+RUN --mount=type=cache,target=/root/.npm npm ci
+
+RUN --mount=type=cache,target=/ms-playwright-cache,sharing=locked \
+    PLAYWRIGHT_BROWSERS_PATH=/ms-playwright-cache \
+    PLAYWRIGHT_DOWNLOAD_HOST=https://npmmirror.com/mirrors/playwright/ \
+    npx playwright install --with-deps chromium \
+  && cp -r /ms-playwright-cache /ms-playwright
+
+COPY apps/backend/subservices/ocr-worker/ ./subservices/ocr-worker/
+RUN --mount=type=cache,target=/root/.cache/pip,sharing=locked \
+    python3 -m pip install --break-system-packages ./subservices/ocr-worker
+
+COPY apps/backend/ ./
+
+RUN mkdir -p /app/data /app/data/browser-sessions /app/data/redeem-screenshots
+
+EXPOSE 3000
+
+CMD ["npm", "run", "start"]