重组为 backend/clients/docs 三层结构，并清理 git 污染。

将后端迁入 backend/，完善根目录 .gitignore，删除误提交的 .mypy_cache 缓存文件。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_baidu_unified_env.py

@@ -0,0 +1,20 @@
+"""百度凭据：仅从环境变量 BAIDU_APP_ID / BAIDU_API_KEY / BAIDU_SECRET_KEY 读入。"""
+
+import os
+from unittest.mock import patch
+
+from app.config import Settings
+
+
+def test_speech_creds_from_baidu_env_triplet() -> None:
+    extra = {
+        "BAIDU_APP_ID": "app-x",
+        "BAIDU_API_KEY": "key-x",
+        "BAIDU_SECRET_KEY": "sec-x",
+    }
+    with patch.dict(os.environ, extra, clear=False):
+        s = Settings()
+    assert s.baidu_speech_app_id == "app-x"
+    assert s.baidu_speech_api_key == "key-x"
+    assert s.baidu_speech_secret_key == "sec-x"
+    assert s.baidu_speech_configured is True