refactor: 镜像只装依赖，代码改为挂载

- Dockerfile: 去掉 COPY . .，镜像退化为纯依赖层，改代码不再需要重建镜像
- Dockerfile: 补 libgl1 / libxcb1 / libglib2.0-0 等 X11 库。opencv-python 在导入时需要它们，而 tools/utils.py 会经 slider_util 导入 cv2 —— 缺了不是某个边角功能挂掉，是整个应用起不来（已在真机冒烟中验证）
- Dockerfile: 补 tzdata，否则 TZ=Asia/Shanghai 被静默忽略，所有时间戳落成 UTC
- Dockerfile: apt 与 pip 均改走国内源。deb.debian.org 实测约 13 kB/s，96 MB 构建依赖要跑半小时以上
- docker-compose: 挂载 ./ 到 /app，部署流程从「重建镜像」变成「重传 + 重启」
- main.py: 启动时若缺 api/webui/index.html 就打印警告，避免只返回一段 JSON 却看着一切正常

docker-compose.yml

@@ -19,6 +19,9 @@ services:
       TZ: Asia/Shanghai
 
     volumes:
-      # Per-run crawler output, read back by the monitor layer. Without this the
-      # data would live inside the container and vanish on every rebuild.
-      - ./data:/app/data
+      # The code is mounted rather than baked in, so shipping a change is
+      # "re-upload the tarball, restart" instead of an image rebuild. Only the
+      # dependencies live in the image, because those are the expensive part and
+      # they change rarely -- rebuild only when requirements.txt or the
+      # Dockerfile itself changes.
+      - ./:/app