fix: video size error handling — validation, timeouts, failure surfacing

Backend:
- Increase ffmpeg/ffprobe timeouts from 15s to 60s for large videos
- Classify processing errors (timeout, OOM, corrupt) with user-facing messages
- Emit failure details via media:job:update socket event (error field)
- Bump refboard container memory 512M → 1G

Frontend:
- Client-side file size validation (200MB) before upload starts
- Oversized files show immediate error in upload manager, skip upload
- Handle media:job:update status='failed' — surface error in upload panel
- Add processingFailed() to UploadManager for video processing errors

frontend/src/hooks/useCanvasSetup.ts

@@ -184,10 +184,18 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
 
         // Media processing pipeline: upgrade videos when poster/metadata arrives
         socket.on('media:job:update', (data: any) => {
-          if (!data || data.status !== 'done') return;
-          const { imageId, posterAssetKey, nativeWidth, nativeHeight, duration } = data;
+          if (!data) return;
+          const { imageId, status } = data;
           if (!imageId) return;
 
+          // Surface processing failures to upload manager
+          if (status === 'failed') {
+            uploadManager.processingFailed(imageId, data.error || 'Video processing failed');
+            return;
+          }
+          if (status !== 'done') return;
+          const { posterAssetKey, nativeWidth, nativeHeight, duration } = data;
+
           // Update upload manager — transitions video jobs from "processing" to "done"
           uploadManager.processingComplete(imageId);