Add camera/webcam capture engine with engine-aware display picker

- New CameraEngine using OpenCV VideoCapture for webcam capture
- HAS_OWN_DISPLAYS class attribute on CaptureEngine base to distinguish
  engines with their own device lists from desktop monitor engines
- Display picker renders device list for cameras/scrcpy, spatial layout
  for desktop monitors
- Engine-aware display label formatting (camera name vs monitor index)
- Stream modal properly loads engine-specific displays on template change,
  edit, and clone
- Camera backend config rendered as dropdown (auto/dshow/msmf/v4l2)
- Remove offline label from device cards (healthcheck indicator suffices)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/pyproject.toml

@@ -57,6 +57,9 @@ dev = [
     "black>=24.0.0",
     "ruff>=0.6.0",
 ]
+camera = [
+    "opencv-python-headless>=4.8.0",
+]
 # High-performance screen capture engines (Windows only)
 perf = [
     "dxcam>=0.0.5; sys_platform == 'win32'",