video/render: strict Zod schema before upstream forward

[Flotapponnier]

Hotfix cherry-pick to main. Same change as #1216 (dev).

Summary

• Adds a strict Zod schema on /api/video/render body before forwarding to the Remotion renderer.
• Caps array sizes (100 providers, 2000 timestamps/values) to prevent renderer OOM/CPU-bomb from a hostile POST.

Test plan

• Frontend export-video modal still returns a video url on happy path
• Malformed body returns 400 bad_body