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/Controllers/SceneColorController.cs

@@ -0,0 +1,87 @@
+using FlatRender.ContentSvc.Application.Services;
+using FlatRender.ContentSvc.Models;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+namespace FlatRender.ContentSvc.Controllers;
+
+[ApiController]
+[Route("v1/scenes")]
+public class ScenesController(SceneColorService svc) : ControllerBase
+{
+    [HttpGet]
+    public async Task<IActionResult> List([FromQuery(Name = "project_id")] Guid projectId) =>
+        Ok(await svc.GetScenesAsync(projectId));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] CreateSceneRequest req) =>
+        Ok(await svc.CreateSceneAsync(req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPut("{id:guid}")]
+    public async Task<IActionResult> Update(Guid id, [FromBody] UpdateSceneRequest req) =>
+        Ok(await svc.UpdateSceneAsync(id, req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpDelete("{id:guid}")]
+    public async Task<IActionResult> Delete(Guid id)
+    {
+        await svc.DeleteSceneAsync(id);
+        return NoContent();
+    }
+}
+
+[ApiController]
+[Route("v1/shared-colors")]
+public class SharedColorsController(SceneColorService svc) : ControllerBase
+{
+    [HttpGet]
+    public async Task<IActionResult> List([FromQuery(Name = "project_id")] Guid projectId) =>
+        Ok(await svc.GetSharedColorsAsync(projectId));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] SaveSharedColorRequest req) =>
+        Ok(await svc.CreateSharedColorAsync(req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPut("{id:guid}")]
+    public async Task<IActionResult> Update(Guid id, [FromBody] SaveSharedColorRequest req) =>
+        Ok(await svc.UpdateSharedColorAsync(id, req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpDelete("{id:guid}")]
+    public async Task<IActionResult> Delete(Guid id)
+    {
+        await svc.DeleteSharedColorAsync(id);
+        return NoContent();
+    }
+}
+
+[ApiController]
+[Route("v1/color-presets")]
+public class ColorPresetsController(SceneColorService svc) : ControllerBase
+{
+    [HttpGet]
+    public async Task<IActionResult> List([FromQuery(Name = "project_id")] Guid projectId) =>
+        Ok(await svc.GetColorPresetsAsync(projectId));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] SaveColorPresetRequest req) =>
+        Ok(await svc.CreateColorPresetAsync(req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPut("{id:guid}")]
+    public async Task<IActionResult> Update(Guid id, [FromBody] SaveColorPresetRequest req) =>
+        Ok(await svc.UpdateColorPresetAsync(id, req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpDelete("{id:guid}")]
+    public async Task<IActionResult> Delete(Guid id)
+    {
+        await svc.DeleteColorPresetAsync(id);
+        return NoContent();
+    }
+}