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/config.py

@@ -39,6 +39,29 @@ def build_rtsp_url(
     return "rtsp://{0}:{1}@{2}:{3}{4}".format(user, secret, host, int(port), path)
 
 
+def build_ffmpeg_options(
+    transport: str = "tcp",
+    timeout_seconds: float = 5.0,
+    low_latency: bool = True,
+) -> str:
+    """Build the OpenCV FFmpeg capture options string from friendly settings.
+
+    OpenCV reads ``OPENCV_FFMPEG_CAPTURE_OPTIONS`` when the stream is opened, so
+    the caller must set this into the environment before ``cv2.VideoCapture``.
+    Format is ``key;value`` pairs joined by ``|``.
+    """
+
+    if transport not in ("tcp", "udp"):
+        raise ValueError("transport must be 'tcp' or 'udp'")
+    parts = ["rtsp_transport;{0}".format(transport)]
+    if timeout_seconds and float(timeout_seconds) > 0:
+        parts.append("stimeout;{0}".format(int(float(timeout_seconds) * 1_000_000)))
+    if low_latency:
+        parts.append("fflags;nobuffer")
+        parts.append("flags;low_delay")
+    return "|".join(parts)
+
+
 @dataclass(frozen=True)
 class EventConfig:
     keypoint_confidence_threshold: float
@@ -58,6 +81,15 @@ class AppConfig:
     event: EventConfig
     event_dir: Path
     source_mode: str = "stream"
+    transport: str = "tcp"
+    timeout_seconds: float = 5.0
+    low_latency: bool = True
+
+    @property
+    def ffmpeg_capture_options(self) -> str:
+        """OpenCV FFmpeg options string to set before opening the stream."""
+
+        return build_ffmpeg_options(self.transport, self.timeout_seconds, self.low_latency)
 
     @property
     def runtime_config_version(self) -> str:
@@ -123,6 +155,9 @@ def write_local_camera_source(
     username: str,
     password: str,
     mode: str = "stream",
+    transport: str = "tcp",
+    timeout_seconds: float = 5.0,
+    low_latency: bool = True,
 ) -> Path:
     """Persist a structured camera source into an untracked local config file.
 
@@ -141,6 +176,9 @@ def write_local_camera_source(
         "username": str(username),
         "password": str(password),
         "mode": mode,
+        "transport": transport,
+        "timeout_seconds": float(timeout_seconds),
+        "low_latency": bool(low_latency),
     }
     path.write_text(json.dumps(data, ensure_ascii=False, indent=2) + "\n", encoding="utf-8")
     return path
@@ -189,6 +227,16 @@ def load_config(path: Path) -> AppConfig:
     if source_mode not in ("replay", "stream"):
         raise ConfigError("source.mode must be 'replay' or 'stream'")
 
+    transport = str(source.get("transport", "tcp")).strip().lower()
+    if transport not in ("tcp", "udp"):
+        raise ConfigError("source.transport must be 'tcp' or 'udp'")
+    timeout_seconds = _number(
+        source.get("timeout_seconds", 5.0), "source.timeout_seconds", 0.0, 60.0
+    )
+    low_latency = source.get("low_latency", True)
+    if not isinstance(low_latency, bool):
+        raise ConfigError("source.low_latency must be a boolean")
+
     model = _mapping(root.get("model"), "model")
     model_sha256 = _text(model.get("sha256"), "model.sha256").lower()
     if not _SHA256.match(model_sha256):
@@ -240,4 +288,7 @@ def load_config(path: Path) -> AppConfig:
         event=event,
         event_dir=_resolve_path(config_path, artifacts.get("event_dir"), "artifacts.event_dir"),
         source_mode=source_mode,
+        transport=transport,
+        timeout_seconds=timeout_seconds,
+        low_latency=low_latency,
     )