data.background is ignored by /render-onserver

Setting a background colour on a video project has no effect on the headless render. No error, no warning — the value is silently dropped.

Steps to reproduce
1. Set data.background on a video project to any colour
2. Render via /render-onserver
3. Sample any pixel of the stage that no layer covers

Expected
The stage renders in the colour set on data.background.

Actual
The stage comes back rgb(2,2,2) regardless of what is set.

Context
Workaround is to put a full-frame rectangle shape on the bottom channel spanning the whole duration. Worth either honouring the field or removing it, since it currently reads as supported.

Environment
App version: 8.8.0
Platform: Windows 11 Pro 10.0.26200