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/Models/SceneColor.cs

@@ -0,0 +1,50 @@
+namespace FlatRender.ContentSvc.Models;
+
+// ── Scenes (project-scoped scene templates) ──────────────────────────────────
+
+public record SceneResponse(
+    Guid Id, Guid ProjectId, string Key, string Title, string? LocalizedTitle,
+    string SceneType, string? Image, string? Demo, string? SceneColorSvg, string? SnapshotUrl,
+    bool GenerateKf, decimal? DefaultDurationSec, decimal? MinDurationSec, decimal? MaxDurationSec,
+    decimal OverlapAtEndSec, bool CanHandleDuration, bool ManualColorSelection, int Sort, bool IsActive
+);
+
+public record CreateSceneRequest(
+    Guid ProjectId, string Key, string Title, string? LocalizedTitle, string? SceneType,
+    string? Image, string? Demo, string? SceneColorSvg, string? SnapshotUrl, bool GenerateKf,
+    decimal? DefaultDurationSec, decimal? MinDurationSec, decimal? MaxDurationSec, decimal OverlapAtEndSec,
+    bool CanHandleDuration, bool ManualColorSelection, int Sort, bool IsActive
+);
+
+public record UpdateSceneRequest(
+    string Key, string Title, string? LocalizedTitle, string? SceneType,
+    string? Image, string? Demo, string? SceneColorSvg, string? SnapshotUrl, bool GenerateKf,
+    decimal? DefaultDurationSec, decimal? MinDurationSec, decimal? MaxDurationSec, decimal OverlapAtEndSec,
+    bool CanHandleDuration, bool ManualColorSelection, int Sort, bool IsActive
+);
+
+// ── Shared colors (project-scoped named colors) ──────────────────────────────
+
+public record SharedColorResponse(
+    Guid Id, Guid ProjectId, string ElementKey, string Title, string? Icon,
+    string AttrValue, string DefaultColor, int Sort
+);
+
+public record SaveSharedColorRequest(
+    Guid ProjectId, string ElementKey, string Title, string? Icon,
+    string? AttrValue, string DefaultColor, int Sort
+);
+
+// ── Color presets (named palettes of colors) ─────────────────────────────────
+
+public record ColorPresetItemResponse(Guid Id, string ElementKey, string Value, int Sort);
+
+public record ColorPresetResponse(
+    Guid Id, Guid ProjectId, string? Name, int Sort, List<ColorPresetItemResponse> Items
+);
+
+public record ColorPresetItemInput(string ElementKey, string Value, int Sort);
+
+public record SaveColorPresetRequest(
+    Guid ProjectId, string? Name, int Sort, List<ColorPresetItemInput> Items
+);