feat(admin): manually edit scene inputs (content elements)

Scene content elements (the editable Text/Media/Color/… inputs inside a scene)
had no CRUD — only AEP-import created them, so admins couldn't define or edit
them. Added full management:

content-svc:
- SceneElementsController: GET/POST/PUT/DELETE /v1/scene-elements?scene_id=
- SceneColorService: Get/Create/Update/DeleteContentElementAsync
- ContentElementResponse + SaveContentElementRequest (key, title, type,
  default_value, hint, position, text-box/font/media flags)
gateway: route /v1/scene-elements/*path → content
frontend: SceneColorEditor scenes tab → per-scene "ورودی‌ها" expander with full
  add/edit/delete of inputs (15 element types: Text/Media/Color/Number/…)

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

services/content/FlatRender.ContentSvc/Models/SceneColor.cs

@@ -48,3 +48,21 @@ public record ColorPresetItemInput(string ElementKey, string Value, int Sort);
 public record SaveColorPresetRequest(
     Guid ProjectId, string? Name, int Sort, List<ColorPresetItemInput> Items
 );
+
+// ── Scene content elements (the editable inputs inside a scene) ───────────────
+
+public record ContentElementResponse(
+    Guid Id, Guid SceneId, string Key, string Title, string? Hint,
+    string Type, string? DefaultValue, int PositionInContainer,
+    bool IsTextBox, int? MaxSize, int? FontSize, bool IsFontChangeable,
+    bool IsFontSizeChangeable, bool VideoSupport, int? Width, int? Height,
+    string? Thumbnail
+);
+
+public record SaveContentElementRequest(
+    Guid SceneId, string Key, string Title, string? Hint,
+    string Type, string? DefaultValue, int PositionInContainer,
+    bool IsTextBox, int? MaxSize, int? FontSize,
+    bool IsFontChangeable, bool IsFontSizeChangeable,
+    bool VideoSupport, int? Width, int? Height, string? Thumbnail
+);