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.example.json

@@ -2,7 +2,10 @@
   "source": {
     "id": "lobby-camera-01",
     "rtsp_url_env": "SILVER_POSE_RTSP_URL",
-    "mode": "stream"
+    "mode": "stream",
+    "transport": "tcp",
+    "timeout_seconds": 5.0,
+    "low_latency": true
   },
   "model": {
     "path": "models/best.pt",