📦 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/core-js/modules/esnext.observable.from.js

@@ -0,0 +1,38 @@
+'use strict';
+var $ = require('../internals/export');
+var getBuiltIn = require('../internals/get-built-in');
+var call = require('../internals/function-call');
+var anObject = require('../internals/an-object');
+var isConstructor = require('../internals/is-constructor');
+var getIterator = require('../internals/get-iterator');
+var getIteratorMethod = require('../internals/get-iterator-method');
+var getMethod = require('../internals/get-method');
+var iterate = require('../internals/iterate');
+var wellKnownSymbol = require('../internals/well-known-symbol');
+
+var $$OBSERVABLE = wellKnownSymbol('observable');
+
+// `Observable.from` method
+// https://github.com/tc39/proposal-observable
+$({ target: 'Observable', stat: true, forced: true }, {
+  from: function from(x) {
+    var C = isConstructor(this) ? this : getBuiltIn('Observable');
+    var observableMethod = getMethod(anObject(x), $$OBSERVABLE);
+    if (observableMethod) {
+      var observable = anObject(call(observableMethod, x));
+      return observable.constructor === C ? observable : new C(function (observer) {
+        return observable.subscribe(observer);
+      });
+    }
+    var iteratorMethod = getIteratorMethod(x);
+    // validate that x is iterable synchronously during `from()` call
+    if (!iteratorMethod) getIterator(x);
+    return new C(function (observer) {
+      iterate(getIterator(x, iteratorMethod), function (it, stop) {
+        observer.next(it);
+        if (observer.closed) return stop();
+      }, { IS_ITERATOR: true, INTERRUPTED: true });
+      observer.complete();
+    });
+  }
+});