refactor: make OpenCV an optional dependency

Camera engine and video stream support now gracefully degrade when
opencv-python-headless is not installed. The app starts fine without
it — camera engine simply doesn't register and video streams raise
a clear ImportError with install instructions.

Saves ~45MB for users who don't need camera/video capture.

server/src/wled_controller/core/capture_engines/__init__.py

@@ -12,15 +12,22 @@ from wled_controller.core.capture_engines.dxcam_engine import DXcamEngine, DXcam
 from wled_controller.core.capture_engines.bettercam_engine import BetterCamEngine, BetterCamCaptureStream
 from wled_controller.core.capture_engines.wgc_engine import WGCEngine, WGCCaptureStream
 from wled_controller.core.capture_engines.scrcpy_engine import ScrcpyEngine, ScrcpyCaptureStream
-from wled_controller.core.capture_engines.camera_engine import CameraEngine, CameraCaptureStream
 from wled_controller.core.capture_engines.demo_engine import DemoCaptureEngine, DemoCaptureStream
 
+# Camera engine requires OpenCV — optional dependency
+try:
+    from wled_controller.core.capture_engines.camera_engine import CameraEngine, CameraCaptureStream
+    _has_camera = True
+except ImportError:
+    _has_camera = False
+
 # Auto-register available engines
 EngineRegistry.register(MSSEngine)
 EngineRegistry.register(DXcamEngine)
 EngineRegistry.register(BetterCamEngine)
 EngineRegistry.register(WGCEngine)
 EngineRegistry.register(ScrcpyEngine)
+if _has_camera:
     EngineRegistry.register(CameraEngine)
 EngineRegistry.register(DemoCaptureEngine)
 
@@ -40,8 +47,9 @@ __all__ = [
     "WGCCaptureStream",
     "ScrcpyEngine",
     "ScrcpyCaptureStream",
-    "CameraEngine",
-    "CameraCaptureStream",
     "DemoCaptureEngine",
     "DemoCaptureStream",
 ]
+
+if _has_camera:
+    __all__ += ["CameraEngine", "CameraCaptureStream"]

server/src/wled_controller/core/processing/live_stream_manager.py

@@ -22,7 +22,11 @@ from wled_controller.core.processing.live_stream import (
     ScreenCaptureLiveStream,
     StaticImageLiveStream,
 )
+try:
     from wled_controller.core.processing.video_stream import VideoCaptureLiveStream
+    _has_video = True
+except ImportError:
+    _has_video = False
 from wled_controller.utils import get_logger
 
 logger = get_logger(__name__)
@@ -264,8 +268,13 @@ class LiveStreamManager:
                 logger.warning(f"Skipping unknown filter '{fi.filter_id}': {e}")
         return resolved
 
-    def _create_video_live_stream(self, config) -> VideoCaptureLiveStream:
+    def _create_video_live_stream(self, config):
         """Create a VideoCaptureLiveStream from a VideoCaptureSource config."""
+        if not _has_video:
+            raise ImportError(
+                "OpenCV is required for video stream support. "
+                "Install it with: pip install opencv-python-headless"
+            )
         stream = VideoCaptureLiveStream(
             url=config.url,
             loop=config.loop,

server/src/wled_controller/core/processing/video_stream.py

@@ -9,9 +9,14 @@ import threading
 import time
 from typing import Optional
 
-import cv2
 import numpy as np
 
+try:
+    import cv2
+    _has_cv2 = True
+except ImportError:
+    _has_cv2 = False
+
 from wled_controller.core.capture_engines.base import ScreenCapture
 from wled_controller.core.processing.live_stream import LiveStream
 from wled_controller.utils import get_logger
@@ -67,12 +72,22 @@ def resolve_youtube_url(url: str, resolution_limit: Optional[int] = None) -> str
         return stream_url
 
 
+def _require_cv2():
+    """Raise a clear error if OpenCV is not installed."""
+    if not _has_cv2:
+        raise ImportError(
+            "OpenCV is required for camera and video support. "
+            "Install it with: pip install opencv-python-headless"
+        )
+
+
 def extract_thumbnail(url: str, resolution_limit: Optional[int] = None) -> Optional[np.ndarray]:
     """Extract the first frame of a video as a thumbnail (RGB numpy array).
 
     For YouTube URLs, resolves via yt-dlp first.
     Returns None on failure.
     """
+    _require_cv2()
     try:
         actual_url = url
         if is_youtube_url(url):
@@ -127,6 +142,7 @@ class VideoCaptureLiveStream(LiveStream):
         resolution_limit: Optional[int] = None,
         target_fps: int = 30,
     ):
+        _require_cv2()
         self._original_url = url
         self._resolved_url: Optional[str] = None
         self._loop = loop
@@ -136,7 +152,7 @@ class VideoCaptureLiveStream(LiveStream):
         self._resolution_limit = resolution_limit
         self._target_fps = target_fps
 
-        self._cap: Optional[cv2.VideoCapture] = None
+        self._cap = None  # Optional[cv2.VideoCapture]
         self._video_fps: float = 30.0
         self._total_frames: int = 0
         self._video_duration: float = 0.0