fix: stabilize cover extraction

backend/app/jobs/media_process_job.go

@@ -92,9 +92,13 @@ func (j *MediaProcessWorker) Work(ctx context.Context, job *river.Job[args.Media
 					"-i",
 					inputFile,
 					"-ss",
-					"00:00:01.000",
+					"00:00:00.000",
 					"-vframes",
 					"1",
+					"-vf",
+					"format=yuv420p",
+					"-update",
+					"1",
 					coverFile,
 				)
 				if out, err := cmd.CombinedOutput(); err != nil {