fix: send video duration as string, not integer

The Kling API expects duration as '5' or '10' (string), not an int.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

freepik_cli/api/videos.py

@@ -35,7 +35,7 @@ class VideoAPI:
         if prompt:
             payload["prompt"] = prompt
         if duration:
-            payload["duration"] = duration
+            payload["duration"] = str(duration)
         if aspect_ratio:
             payload["aspect_ratio"] = aspect_ratio
         if seed is not None: