feat(video): add removeBackground prop for chroma key compositing

[caffeinum]

Summary

• adds removeBackground prop to <Video> component for green screen / chroma key support
• works with prompt mode only - instructs AI to generate solid color background, then removes it with ffmpeg colorkey filter
• supports custom color, tolerance, and blend options

Usage

// simple - default green #00FF00
<Video prompt="person waving" removeBackground />

// custom options
<Video 
  prompt="presenter speaking"
  removeBackground={{ 
    color: '#00FF00',
    tolerance: 0.15,
    blend: 0.05
  }}
/>

// in pip context
<Clip>
  <Video src="background.mp4" />
  <Overlay left="70%" top="10%" width="25%">
    <Video prompt="person talking" removeBackground />
  </Overlay>
</Clip>

Changes

• src/react/types.ts - added RemoveBackgroundOptions interface and prop
• src/react/renderers/video.ts - prompt modification + colorkey filter application
• src/providers/ffmpeg.ts - added applyColorKey() method
• src/react/index.ts - exported RemoveBackgroundOptions type
• src/react/examples/remove-background-demo.tsx - example

closes #38