backend_design.txt

The backend will be a Python Flask server with the following key components:

1. Flask app setup:
   - Import necessary libraries (Flask, FFmpeg-python, os, etc.)
   - Initialize Flask app

2. File upload route (/upload):
   - Accept POST requests with video file
   - Save uploaded file temporarily
   - Return a unique job ID

3. Conversion route (/convert):
   - Accept POST requests with job ID, desired speed, and resolution
   - Use FFmpeg-python to convert the video:
     - Change format to MP4
     - Adjust playback speed
     - Resize to selected resolution
   - Save converted video
   - Return download link

4. Download route (/download/<job_id>):
   - Serve the converted video file

5. Status route (/status/<job_id>):
   - Return current status of conversion job

6. Cleanup function:
   - Periodically remove old temporary and converted files

The server will use FFmpeg-python for video processing, which allows for easy manipulation of video properties like format, speed, and resolution.