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

api/main.py

@@ -70,6 +70,16 @@ async def lifespan(_app: FastAPI):
 
     await init_db()
 
+    # The WebUI bundle is gitignored and built separately, so a deployment that
+    # forgot it would otherwise come up looking healthy and serve a bare JSON
+    # stub at "/" -- worth one loud line at boot rather than a puzzled operator.
+    if not os.path.exists(os.path.join(WEBUI_DIR, "index.html")):
+        print(
+            "[综合采集平台] 警告：未找到前端产物 api/webui/index.html，"
+            "根路径只会返回一段 JSON。请先在 webui/ 下执行 npm run build。",
+            flush=True,
+        )
+
     generated = await ensure_initial_credential()
     if generated:
         # Printed once, on the run that creates it. There is no unauthenticated