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/server.js

@@ -174,6 +174,14 @@ async function start() {
     console.error('[server] MM watcher failed to start:', err.message);
   }
 
+  // Start media processing worker
+  try {
+    const { startMediaWorker } = require('./services/media-worker');
+    startMediaWorker(io);
+  } catch (err) {
+    console.error('[server] Media worker failed to start:', err.message);
+  }
+
   server.listen(PORT, '0.0.0.0', () => {
     console.log(`[server] RefBoard backend listening on port ${PORT}`);
   });
@@ -188,6 +196,11 @@ function shutdown(signal) {
     stopWatcher();
   } catch {}
 
+  try {
+    const { stopMediaWorker } = require('./services/media-worker');
+    stopMediaWorker();
+  } catch {}
+
   io.close(() => {
     console.log('[server] Socket.IO closed');
   });