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_stream_worker_redaction.py

@@ -0,0 +1,18 @@
+from app.services.video.rtsp_redact import redact_rtsp_url
+
+
+def test_redact_rtsp_url_with_credentials() -> None:
+    assert (
+        redact_rtsp_url("rtsp://admin:secret@10.0.0.1:554/Streaming/Channels/101")
+        == "rtsp://***@10.0.0.1:554/Streaming/Channels/101"
+    )
+
+
+def test_redact_rtsp_url_without_credentials_unchanged() -> None:
+    url = "rtsp://10.0.0.1:554/Streaming/Channels/101"
+    assert redact_rtsp_url(url) == url
+
+
+def test_redact_rtsp_url_empty() -> None:
+    assert redact_rtsp_url(None) == ""
+    assert redact_rtsp_url("") == ""