Add fade in and fade out effects to video

[harry0703]

Related to #374

Implements fade in and fade out effects to the video generation process to prevent an abrupt ending.

• Adds fade in and fade out effects to the generate_video function in app/services/video.py using moviepy.editor's fadein and fadeout methods with a duration of 1 second for both effects. This change ensures a smoother transition at the beginning and end of the video.

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