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/stores/uploadManager.ts

@@ -88,6 +88,18 @@ export class UploadManager {
     }
   }
 
+  /** Video processing failed (from media:job:update socket event with status='failed'). */
+  processingFailed(imageId: string, error: string) {
+    for (const job of this.jobs.values()) {
+      if (job.imageId === imageId && (job.status === 'processing' || job.status === 'uploading')) {
+        job.status = 'failed';
+        job.error = error;
+        this._notify();
+        return;
+      }
+    }
+  }
+
   /** Mark a job as failed. */
   setFailed(jobId: string, error: string) {
     const job = this.jobs.get(jobId);