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_algorithm_cli_modes.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+from app.algorithm_runner.cli import _resolve_source_mode, _stream_time_for_frame
+
+
+def test_resolve_source_mode_auto_detects_local_video(tmp_path) -> None:
+    video = tmp_path / "case.mp4"
+    video.write_bytes(b"")
+
+    assert _resolve_source_mode(str(video), "auto") == "file"
+
+
+def test_resolve_source_mode_auto_keeps_rtsp_realtime() -> None:
+    assert _resolve_source_mode("rtsp://127.0.0.1:8554/demo", "auto") == "realtime"
+
+
+def test_resolve_source_mode_respects_explicit_choice() -> None:
+    assert _resolve_source_mode("rtsp://127.0.0.1:8554/demo", "file") == "file"
+
+
+def test_realtime_stream_time_uses_frame_index_not_wall_clock() -> None:
+    assert _stream_time_for_frame(
+        source_mode="realtime",
+        cap_pos_msec=999_999.0,
+        frame_idx=50,
+        fps_hint=25.0,
+    ) == 2.0
+
+
+def test_file_stream_time_prefers_capture_timestamp() -> None:
+    assert _stream_time_for_frame(
+        source_mode="file",
+        cap_pos_msec=1234.0,
+        frame_idx=50,
+        fps_hint=25.0,
+    ) == 1.234