refactor(backend): 启动环境初始化与资源项目化（零跨项目依赖）

- main.py：uv run main.py 一键启动——读取 backend/.env、TORCH_HOME/NLTK_DATA 指向项目内缓存、certifi SSL + NO_PROXY 网络直连、默认开启 s2s，启动横幅
- vendor/：补丁后 s2s 云化栈移入项目（backend/vendor/s2s-cloud），pyproject sources 改相对路径
- .gitignore：忽略 .torch-cache / nltk_data 缓存
- AGENTS.md：进程管理铁律（AI 不启动/重启服务，由用户操作）

backend/vendor/s2s-cloud/Dockerfile.arm64

@@ -0,0 +1,28 @@
+ARG BASE_PLATFORM=linux/arm64
+FROM --platform=$BASE_PLATFORM nvidia/cuda:12.8.1-cudnn-runtime-ubuntu24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+ENV PYTHONUNBUFFERED=1
+ENV PATH="/usr/src/app/.venv/bin:${PATH}"
+
+WORKDIR /usr/src/app
+
+# Install packages
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        ca-certificates \
+        git \
+        libportaudio2 \
+        libsndfile1 \
+        python3 \
+        python3-pip \
+        python3-venv \
+    && rm -rf /var/lib/apt/lists/*
+RUN python3 -m pip install --no-cache-dir --break-system-packages uv
+
+COPY pyproject.toml README.md LICENSE MANIFEST.in ./
+RUN uv sync --python /usr/bin/python3 --no-install-project --no-dev
+
+COPY . .
+RUN uv sync --python /usr/bin/python3 --no-dev
+RUN python -c "import nltk; nltk.download('punkt_tab'); nltk.download('averaged_perceptron_tagger_eng')"