feat: media processing pipeline, spatial indexing, canvas-based video rendering

- Background media worker: polls media_jobs table, runs ffprobe+ffmpeg
  with concurrency limit, generates video posters, emits socket events
- Non-blocking video upload: stores file + enqueues job, returns immediately
- Poster hydration on board load: GET /boards/:id injects poster/dimensions
  from DB into canvas_state video objects
- SpatialGrid: fixed-cell (512px) spatial hash for O(nearby) culling instead
  of O(all) item scanning, eliminates setTimeout violations
- Canvas-based video rendering: draws video frames to offscreen canvas then
  uploads to GPU, completely eliminates GL_INVALID_OPERATION errors from
  PixiJS VideoSource auto-update mechanism
- Server poster upgrade path: culling ticker and applyProcessedMedia() both
  upgrade client-captured posters to server posters when available
- Pause restores server poster (paused video behaves like an image)
- Selection drag-end persistence: onObjectDragEnd broadcasts + saves + undo
- Live media:job:update socket handler patches scene data + VideoSprite
  dimensions without broadcast fanout

backend/routes/boards.js

@@ -120,10 +120,33 @@ router.get('/:boardId', (req, res) => {
 
     const images = getBoardImages(board.id);
 
+    // Hydrate video objects with poster/dimensions from DB
+    // (worker may have finished after canvas was last saved)
+    let canvasState = board.canvas_state ? JSON.parse(board.canvas_state) : {};
+    if (canvasState.objects && Array.isArray(canvasState.objects)) {
+      const videoImages = new Map();
+      for (const img of images) {
+        if (img.media_type === 'video' && img.poster_asset_key) {
+          videoImages.set(img.asset_key, img);
+        }
+      }
+      if (videoImages.size > 0) {
+        for (const obj of canvasState.objects) {
+          if (obj.type !== 'video') continue;
+          const dbImg = videoImages.get(obj.asset);
+          if (!dbImg) continue;
+          if (!obj.poster && dbImg.poster_asset_key) obj.poster = dbImg.poster_asset_key;
+          if (!obj.nativeW && dbImg.native_width) { obj.nativeW = dbImg.native_width; obj.w = dbImg.native_width; }
+          if (!obj.nativeH && dbImg.native_height) { obj.nativeH = dbImg.native_height; obj.h = dbImg.native_height; }
+          if (!obj.duration && dbImg.duration) obj.duration = dbImg.duration;
+        }
+      }
+    }
+
     return res.json({
       board: {
         ...board,
-        canvas_state: board.canvas_state ? JSON.parse(board.canvas_state) : {},
+        canvas_state: canvasState,
       },
       collection: {
         id: collection.id,