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/SceneManager.ts

@@ -271,7 +271,7 @@ export class SceneManager {
         // Use cached native dimensions if available (avoids re-init just to discover size)
         const vw = vidData.nativeW || vidData.w;
         const vh = vidData.nativeH || vidData.h;
-        const videoSprite = new VideoSprite(vidData.asset, vw, vh, videoUrl);
+        const videoSprite = new VideoSprite(vidData.asset, vw, vh, videoUrl, vidData.poster, this.textures);
         // Apply cached native dims to scene data so display matches
         if (vidData.nativeW && vidData.nativeH) {
           data.w = vw;
@@ -607,6 +607,8 @@ export class SceneManager {
     h: number,
     x: number,
     y: number,
+    posterAssetKey?: string,
+    duration?: number,
   ): SceneItem {
     const data: VideoObject = {
       id: crypto.randomUUID(),
@@ -626,6 +628,10 @@ export class SceneManager {
       asset: assetKey,
       muted: true,
       loop: true,
+      nativeW: w,
+      nativeH: h,
+      poster: posterAssetKey,
+      duration,
     };
 
     this._createItem(data, true);