fix: restore NFT video processing support

[JOY (JOY)]

Summary

• add Xav as the Image.Video backend dependency
• install FFmpeg development and runtime libraries in published images
• add an end-to-end CI smoke test that decodes a generated MP4 and resizes its first frame

Evidence

• Image 0.72 conditionally compiles Image.Video only when Xav is configured
• production Docker image builds locally with Alpine 3.22 and FFmpeg 6.1 libraries
• git diff --check passes

Testing

• local production Docker build
• GitHub dependency build and video smoke test required before merge

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