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/tests/test_config.py

@@ -2,7 +2,7 @@ import json
 
 import pytest
 
-from v1.config import ConfigError, load_config
+from v1.config import ConfigError, build_ffmpeg_options, load_config
 
 
 def _write_config(path, source):
@@ -140,6 +140,58 @@ def test_public_example_config_has_no_embedded_credentials():
     assert "rtsp_url_env" in source
 
 
+def test_build_ffmpeg_options_variants():
+    assert build_ffmpeg_options() == "rtsp_transport;tcp|stimeout;5000000|fflags;nobuffer|flags;low_delay"
+    assert build_ffmpeg_options("udp", 3, False) == "rtsp_transport;udp|stimeout;3000000"
+    with pytest.raises(ValueError):
+        build_ffmpeg_options("http")
+
+
+def test_source_capture_tuning_defaults_to_tcp_low_latency(tmp_path):
+    config_file = tmp_path / "config.json"
+    _write_config(
+        config_file,
+        {"id": "c", "host": "192.0.2.10", "username": "u", "password": "p"},
+    )
+
+    config = load_config(config_file)
+
+    assert config.transport == "tcp"
+    assert config.timeout_seconds == 5.0
+    assert config.low_latency is True
+    assert (
+        config.ffmpeg_capture_options
+        == "rtsp_transport;tcp|stimeout;5000000|fflags;nobuffer|flags;low_delay"
+    )
+
+
+def test_source_capture_tuning_is_parsed(tmp_path):
+    config_file = tmp_path / "config.json"
+    _write_config(
+        config_file,
+        {
+            "id": "c", "host": "192.0.2.10", "username": "u", "password": "p",
+            "transport": "udp", "timeout_seconds": 3, "low_latency": False,
+        },
+    )
+
+    config = load_config(config_file)
+
+    assert config.transport == "udp"
+    assert config.ffmpeg_capture_options == "rtsp_transport;udp|stimeout;3000000"
+
+
+def test_invalid_transport_is_rejected(tmp_path):
+    config_file = tmp_path / "config.json"
+    _write_config(
+        config_file,
+        {"id": "c", "host": "192.0.2.10", "username": "u", "password": "p", "transport": "http"},
+    )
+
+    with pytest.raises(ConfigError, match="transport"):
+        load_config(config_file)
+
+
 def test_source_mode_defaults_to_stream(tmp_path, monkeypatch):
     config_file = tmp_path / "config.json"
     _write_config(