feat(refboard): server-generated video posters via ffmpeg

Upload pipeline now extracts poster frame (JPEG) and metadata
(width, height, duration) from videos at upload time using ffmpeg.

Server:
- Add ffmpeg to Docker image (Alpine)
- video-utils.js: probeVideo() and extractPoster() using ffprobe/ffmpeg
- Upload response includes poster_asset_key and duration

Frontend:
- VideoObject gains poster and duration fields in scene format
- VideoSprite accepts posterAssetKey + TextureManager, loads poster
  as a regular image texture on construction (no <video> needed)
- Server poster loaded/released via TextureManager ref counting
- addVideoFromUpload passes poster and duration through to scene data
- image-drop.ts forwards poster_asset_key from upload response

Result: videos with server posters render as images by default.
Zero <video> elements needed for thumbnails. Only explicit play
creates a media element.

frontend/src/canvas/image-drop.ts

@@ -55,7 +55,9 @@ function handleUploadResult(
   }
 
   if (mediaType === 'video' && assetKey) {
-    sceneManager.addVideoFromUpload(assetKey, finalW, finalH, x, y);
+    const posterKey: string | undefined = imgData.poster_asset_key;
+    const duration: number | undefined = imgData.duration;
+    sceneManager.addVideoFromUpload(assetKey, finalW, finalH, x, y, posterKey, duration);
   } else if (assetKey) {
     sceneManager.addImageFromUpload(assetKey, finalW, finalH, x, y);
   } else {