📦 添加虚拟环境和启动脚本

新增:
- backend/venv/ - Python 虚拟环境
- backend/start.sh - 启动脚本（使用虚拟环境）
- backend/requirements.txt - 依赖列表
- .gitignore - 忽略虚拟环境和缓存文件

说明:
- 每个项目使用独立虚拟环境
- 避免依赖冲突
- 启动脚本自动创建和激活虚拟环境

frontend/node_modules/asynckit/lib/readable_asynckit.js

@@ -0,0 +1,91 @@
+var streamify = require('./streamify.js')
+  , defer     = require('./defer.js')
+  ;
+
+// API
+module.exports = ReadableAsyncKit;
+
+/**
+ * Base constructor for all streams
+ * used to hold properties/methods
+ */
+function ReadableAsyncKit()
+{
+  ReadableAsyncKit.super_.apply(this, arguments);
+
+  // list of active jobs
+  this.jobs = {};
+
+  // add stream methods
+  this.destroy = destroy;
+  this._start  = _start;
+  this._read   = _read;
+}
+
+/**
+ * Destroys readable stream,
+ * by aborting outstanding jobs
+ *
+ * @returns {void}
+ */
+function destroy()
+{
+  if (this.destroyed)
+  {
+    return;
+  }
+
+  this.destroyed = true;
+
+  if (typeof this.terminator == 'function')
+  {
+    this.terminator();
+  }
+}
+
+/**
+ * Starts provided jobs in async manner
+ *
+ * @private
+ */
+function _start()
+{
+  // first argument – runner function
+  var runner = arguments[0]
+    // take away first argument
+    , args   = Array.prototype.slice.call(arguments, 1)
+      // second argument - input data
+    , input  = args[0]
+      // last argument - result callback
+    , endCb  = streamify.callback.call(this, args[args.length - 1])
+    ;
+
+  args[args.length - 1] = endCb;
+  // third argument - iterator
+  args[1] = streamify.iterator.call(this, args[1]);
+
+  // allow time for proper setup
+  defer(function()
+  {
+    if (!this.destroyed)
+    {
+      this.terminator = runner.apply(null, args);
+    }
+    else
+    {
+      endCb(null, Array.isArray(input) ? [] : {});
+    }
+  }.bind(this));
+}
+
+
+/**
+ * Implement _read to comply with Readable streams
+ * Doesn't really make sense for flowing object mode
+ *
+ * @private
+ */
+function _read()
+{
+
+}