chore: 完善全栈 Docker 开发环境与容器基础设施

backend/Dockerfile.dev

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+# ============================================================
+# 后端开发 Dockerfile — FastAPI + Uvicorn 热更新
+# ============================================================
+FROM python:3.13-slim
+
+WORKDIR /app
+
+# 系统依赖（Pillow / cryptography 编译所需）
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    libpq-dev \
+    libffi-dev \
+    libjpeg-dev \
+    zlib1g-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+# 先装 Python 依赖（利用 Docker 缓存层，改代码不重装）
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# 代码通过 docker-compose volumes 挂载，不需要 COPY
+# --reload 监听代码变化自动重启
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000", "--reload"]