feat(video): Short 9:16 export format

[Flotapponnier]

Summary

• Format toggle (Landscape 16:9 / Short 9:16) in the export video modal; format goes into the render POST body
• Preview video swaps to a vertical 9:16 frame for shorts (object-contain, no distortion)
• Renderer side already deployed on the VPS (v16-short-format): native 1080x1920 render via Remotion canvas override, format in the cache key, vertical intro pre-rendered. Validated end-to-end: 1080x1920 output, 20s render.

Notes

Compositions still use their 16:9 layouts inside the vertical canvas (plumbing v1); per-view portrait layouts are the next pass.