Auto-generate colors.css entry from brand asset colors

[rchlfryn]

Summary

During tenant onboarding, the theme/brand colors step is manual — an engineer must add a CSS class to colors.css and add the slug to centerColorMap for OG images.

This could be partially automated by extracting colors from the uploaded brand assets (logo, etc.) and generating the colors.css entry automatically.

Context

From the onboarding provisioning PR — theme creation was identified as a candidate for automation.

Acceptance Criteria

• Extract primary/secondary colors from uploaded brand assets
• Auto-generate a colors.css entry for the new tenant slug — see onboarding.md for colors
• Auto-add the slug to centerColorMap for OG image generation
• Fall back to manual step if color extraction fails