feat(v1): configurable low-latency capture options

Config gains source.transport (tcp/udp), timeout_seconds and low_latency;
build_ffmpeg_options assembles OPENCV_FFMPEG_CAPTURE_OPTIONS and app.py
sets it into the environment before opening the stream, so tuning lives
in config/settings instead of a launch script. Settings tab exposes a
dropdown/number/checkbox rather than the raw option string. 64 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v1/app.py

@@ -35,6 +35,8 @@ class FrameWorker(QtCore.QThread):
         self._stop = False
 
     def run(self) -> None:
+        # OpenCV reads this env var when it opens the stream, so set it first.
+        os.environ["OPENCV_FFMPEG_CAPTURE_OPTIONS"] = self._config.ffmpeg_capture_options
         mode = SourceMode.STREAM if self._config.source_mode == "stream" else SourceMode.REPLAY
         source = VideoSource(self._config.source_url, mode=mode)
         pipeline = FallPipeline.from_config(self._config, self._pose_adapter)
@@ -162,7 +164,8 @@ def _camera_fields(path: Path) -> Dict:
         return {}
     if "host" not in source:
         return {}
-    return {k: source.get(k) for k in ("host", "port", "channel", "username", "password")}
+    fields = ("host", "port", "channel", "username", "password", "transport", "timeout_seconds", "low_latency")
+    return {k: source.get(k) for k in fields if k in source}
 
 
 if __name__ == "__main__":