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/ffmpeg.ts

@@ -249,9 +249,12 @@ export async function concatMp4(part1Path: string, part2Path: string, outputPath
 }
 
 export async function stitchExtension(params: { part1Path: string; part2: Buffer; tmpDir: string }) {
-  const part2Path = join(params.tmpDir, 'part2.mp4')
+  const part2Path = join(params.tmpDir, `part2_${Date.now()}.mp4`)
   const finalPath = join(params.tmpDir, 'final_stitched_video.mp4')
   writeFileSync(part2Path, params.part2)
+  if (!existsSync(part2Path) || !params.part2.length) {
+    throw new Error('Extension segment was missing before stitch')
+  }
   try {
     await xfadeStitch(params.part1Path, part2Path, finalPath)
   } catch (error) {