fix: 增加 uv pip install 超时到 300s

UV_HTTP_TIMEOUT=300 防止大包（numpy/pandas/cryptography）下载超时

backend/Dockerfile

@@ -10,9 +10,9 @@ COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv
 COPY backend/pyproject.toml .
 COPY backend/.env.example .
 
-# Create venv and install deps
-RUN uv venv /opt/venv && \
-    uv pip install --python /opt/venv/bin/python \
+# Create venv and install deps — increase timeout for slow networks
+RUN UV_HTTP_TIMEOUT=300 uv venv /opt/venv && \
+    UV_HTTP_TIMEOUT=300 uv pip install --python /opt/venv/bin/python \
         --index-url https://mirrors.aliyun.com/pypi/simple/ \
         --extra-index-url https://pypi.org/simple/ \
         "fastapi>=0.104.0" \