fix: 配置清华 PyPI 镜像加速 Python 依赖下载

backend/Dockerfile

@@ -14,9 +14,9 @@ RUN apt-get update && apt-get install \
     -y --no-install-recommends && \
     rm -rf /var/lib/apt/lists/*
 
-# Install Python dependencies
+# Install Python dependencies (use Tsinghua mirror for faster download in China)
 COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+RUN pip install --no-cache-dir -r requirements.txt -i https://pypi.tuna.tsinghua.edu.cn/simple --trusted-host pypi.tuna.tsinghua.edu.cn
 
 # Copy project
 COPY . .