Fix NameError: use constant CROSSFADE_DURATION instead of undefined variable

modules/story_reels/services/story_creator.py

@@ -606,14 +606,14 @@ class StoryCreator:
 
             # Transitions
             if i > 0:
-                clip = clip.crossfadein(crossfade_duration)
+                clip = clip.crossfadein(CROSSFADE_DURATION)
             
             clips.append(clip)
             total_video_duration += duration
         
         # Concatenate clips
         # method="compose" is safer for crossfades
-        video = concatenate_videoclips(clips, method="compose", padding=-crossfade_duration)
+        video = concatenate_videoclips(clips, method="compose", padding=-CROSSFADE_DURATION)
         
         # Final safety: match video length to audio exactly
         if abs(video.duration - audio_duration) > 0.1: