feat(refboard): video budget system, lazy lifecycle, incremental sync fixes

Video memory:
- Lazy <video> creation: constructor makes placeholder only, initVideo()
  creates element with preload='metadata' when near viewport
- Tiered budgets: 1 playing / 6 initialized / 4 poster textures max
- Aggressive teardown: offscreen videos lose <video> element, poster,
  and all textures — zero memory for offscreen videos
- Poster textures dropped while playing (don't keep both resident)
- Dimension caching in scene data (nativeW/nativeH) avoids re-init

Server:
- HTTP Range support (206 Partial Content) for video seeking
- Proper end clamping and invalid range rejection (416)

Sync:
- Remove redundant broadcastSceneDebounced() from broadcastTransform()
- Remove duplicate broadcastElements() from drag-end handler

VideoControls:
- Event-driven (timeupdate/play/pause/seeked) instead of rAF polling
- Tracks actual HTMLVideoElement reference, rebinds on init/teardown
- Seek uses ref-based commit on pointerUp, not conditional onChange

frontend/src/hooks/useCanvasSetup.ts

@@ -138,8 +138,7 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
           syncRef.current?.broadcastTransform(item);
         };
         selection.transformBox.onDragEnd = (itemIds) => {
-          syncRef.current?.broadcastElements(itemIds);
-          onCanvasChange(itemIds);
+          onCanvasChange(itemIds); // broadcasts elements + saves + undo
         };
 
         socket.on('user:joined', (data: any) => {