Implement Video Functions

This issue encompasses all the video processing functions that need to be implemented. If you're interested in working on any specific function, please comment with the function name and the issue number.

### Video Functions to Implement:
- [ ] Implement `resize_video` function (#5)
- [x] Implement `add_watermark` function (#6)
- [ ] Implement `convert_video_format` function (#7)
- [ ] Implement `extract_frames` function (#8)
- [ ] Implement `merge_video_clips` function (#9)
- [ ] Implement `compress_video` function (#17)
- [ ] Implement `stabilize_video` function (#19)
- [ ] Implement `generate_thumbnail` function (#22)
- [ ] Implement `analyze_video_colors` function (#24)
- [ ] Implement `overlay_subtitles` function (#15)
- [ ] Implement `resize_image` function (#14)
- [ ] Implement `validate_file_format` function (#13)
- [ ] Implement `calculate_duration` function (#12)
- [ ] Implement `generate_unique_filename` function (#11)
- [x] Implement `check_file_exists` function (#10)

### How to Participate:
1. Comment on this issue with the function you want to work on, including the issue number.
2. Once assigned, you can start implementing the function.