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/demo/ws/codec.js

@@ -0,0 +1,57 @@
+// @ts-check
+/**
+ * Pure, stateless helpers for the WebSocket realtime client: base64 <-> PCM
+ * conversion for the audio frames on the wire, transcript extraction from a
+ * `response.done` payload, and a tiny URL helper. Kept separate from the client
+ * so the protocol/state logic stays readable.
+ */
+
+/** @param {string} url */
+export function trimTrailingSlash(url) {
+  return url.endsWith("/") ? url.slice(0, -1) : url;
+}
+
+/**
+ * Pull the assistant transcript out of a `response.done` payload. The text
+ * lives in `response.output[].content[].transcript` (audio) or `.text`. Used as
+ * the source of truth for interrupted replies, where the dedicated
+ * `*.transcript.done` event may never arrive.
+ * @param {any} response
+ * @returns {string}
+ */
+export function extractResponseTranscript(response) {
+  const output = response?.output;
+  if (!Array.isArray(output)) return "";
+  /** @type {string[]} */
+  const parts = [];
+  for (const item of output) {
+    for (const part of item?.content ?? []) {
+      const text = part?.transcript ?? part?.text;
+      if (typeof text === "string" && text.trim()) parts.push(text.trim());
+    }
+  }
+  return parts.join(" ").trim();
+}
+
+/** @param {ArrayBuffer} buf */
+export function base64FromArrayBuffer(buf) {
+  const bytes = new Uint8Array(buf);
+  // Chunked encoding so we don't blow up the call stack on long buffers.
+  let binary = "";
+  const chunk = 0x8000;
+  for (let i = 0; i < bytes.length; i += chunk) {
+    binary += String.fromCharCode.apply(null, /** @type {number[]} */ (
+      /** @type {unknown} */ (bytes.subarray(i, i + chunk))
+    ));
+  }
+  return btoa(binary);
+}
+
+/** @param {string} b64 */
+export function base64ToBytes(b64) {
+  const binary = atob(b64);
+  const len = binary.length;
+  const out = new Uint8Array(len);
+  for (let i = 0; i < len; i++) out[i] = binary.charCodeAt(i);
+  return out;
+}