Phase 1: Add SiliconFlow provider + DeepSeek-V4-Flash, optimize Dockerfile with Tsinghua mirrors

Dockerfile

@@ -3,6 +3,8 @@ FROM python:3.11-slim
 WORKDIR /app
 
 # 安装系统依赖（含编译bcrypt/cryptography所需的build-essential）
+# Use Tsinghua mirror for faster downloads in China
+RUN sed -i 's|deb.debian.org|mirrors.tuna.tsinghua.edu.cn|g' /etc/apt/sources.list.d/debian.sources 2>/dev/null || true
 RUN apt-get update && apt-get install -y --no-install-recommends \
     curl \
     ca-certificates \
@@ -24,6 +26,8 @@ COPY frontend/package-lock.json /app/frontend/package-lock.json
 RUN pip install --no-cache-dir -r /app/backend/python/requirements.txt -i https://pypi.tuna.tsinghua.edu.cn/simple
 
 # 安装Node.js (用于前端构建) 并构建前端
+# Use Tsinghua mirror for faster downloads in China
+RUN sed -i 's|deb.debian.org|mirrors.tuna.tsinghua.edu.cn|g' /etc/apt/sources.list.d/debian.sources 2>/dev/null || true
 RUN apt-get update && apt-get install -y --no-install-recommends \
     nodejs \
     npm \