Add high-quality GIF export support

[ax2bboud]

This PR introduces the ability for users to download videos directly as optimized GIFs.

Key Changes

• Frontend (index.html):
  • Added a new GIF toggle button to the format selection group.
  • Ensures the format parameter is correctly passed to the API during the download request.
• Backend (app.py):
  • Implemented a new processing branch for the gif format.
  • Integrated ffmpeg for transcoding with a focus on quality and efficiency:
    • Framerate: Set to 15fps for smooth motion without excessive file size.
    • Resolution: Scaled to a maximum width of 480px (aspect-ratio preserved) to keep downloads fast.
    • Quality: Uses a two-pass filter (palettegen and paletteuse) to ensure the GIF uses an optimized color palette for better visual fidelity.
  • Automated cleanup of the intermediate MP4 file once the GIF conversion is complete.

How to Test

1. Paste a video URL (e.g., from YouTube or TikTok).
2. Click the GIF toggle in the format options.
3. Click Fetch, then click Download.
4. Verify that the resulting file is a high-quality .gif.

[ax2bboud]

Ideally, you'd want people to choose quality and fps options but it would take away from the simplicity of the tool.

[averygan]

Thanks for the PR! Logic looks good - A few things before merge:

1. Add a timeout to the ffmpeg call. The download uses timeout=300, but the conversion has none:
   ff_res = subprocess.run(ffmpeg_cmd, capture_output=True, text=True)
   palettegen/paletteuse over a long clip is slow, and a bad input could hang the worker thread indefinitely. Please add timeout=... and handle subprocess.TimeoutExpired like the download branch does

2. Guard against long videos. A 10-min clip at 15fps/480px produces a multi-hundred-MB GIF and a very long encode. Can we cap duration (e.g. reject over N seconds with a clear error, or trim with -t)?

3. Surface the real ffmpeg error. "Failed to create GIF" drops ff_res.stderr. Mirroring the download branch (ff_res.stderr.strip().split("\n")[-1]) would making debugging easier

[ax2bboud]

Hey @averygan ! Sorry for the late reply. I've made the changes you suggested. A few considerations:

• This requires ffprobe on PATH (ships with the ffmpeg package on most distros, but if the deploy image only has ffmpeg, the probe will silently fail-open and the duration cap won't apply.
• 60s is my pick for the cap; easy to tune via the MAX_GIF_DURATION constant.