Keep extension chains alive and save each finished segment.

The overall bar made a long queue look stuck at ~9% during the first clip, and nothing was written until the last step. Save checkpoints, use per-step progress, and stop treating a short Comfy history lag as a dropped job.

server/utils/jobs.ts

@@ -23,6 +23,7 @@ export interface JobEvent {
   chainStep?: number
   chainTotal?: number
   chainLabel?: string
+  overallProgress?: number
 }
 
 export interface Job {