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

frontend/src/pages/Editor.tsx

@@ -111,6 +111,14 @@ export default function Editor({ isPublicView }: EditorProps) {
     if (changedIds && changedIds.length > 0) {
       // Incremental: broadcast only changed elements
       syncRef.current?.broadcastElements(changedIds);
+      // Keep spatial index in sync for all local mutations (align/flip/arrange/etc.)
+      const scene = canvasRef.current?.getScene();
+      if (scene) {
+        for (const id of changedIds) {
+          const item = scene.items.get(id);
+          if (item) scene.updateSpatialEntry(item);
+        }
+      }
     }
     // Full scene sync happens via debounced SceneManager.onChange chain in sync.ts
     if (undoRef.current && !undoRef.current.isLocked()) {