feat(content): scenes + shared-colors + colour-presets endpoints

Completes the content backend for the studio building blocks (all project-scoped):
- GET /v1/scenes?project_id= + POST/PUT/DELETE (scene metadata CRUD)
- GET /v1/shared-colors?project_id= + POST/PUT/DELETE
- GET /v1/color-presets?project_id= + POST/PUT/DELETE (palette + items)
SceneColorService + DTOs; reads open, writes [Authorize(Roles=Admin)].
Gateway routes /v1/{scenes,shared-colors,color-presets}/* → content.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/content/FlatRender.ContentSvc/Program.cs

@@ -64,6 +64,7 @@ builder.Services.AddScoped<TaxonomyService>();
 builder.Services.AddScoped<TemplateService>();
 builder.Services.AddScoped<CmsService>();
 builder.Services.AddScoped<AiContentService>();
+builder.Services.AddScoped<SceneColorService>();
 
 // HTTP client for the OpenAI-compatible AI provider (base URL is per-tenant config).
 builder.Services.AddHttpClient("openai");