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/Controllers/SceneColorController.cs

@@ -32,6 +32,33 @@ public class ScenesController(SceneColorService svc) : ControllerBase
     }
 }
 
+[ApiController]
+[Route("v1/scene-elements")]
+public class SceneElementsController(SceneColorService svc) : ControllerBase
+{
+    [HttpGet]
+    public async Task<IActionResult> List([FromQuery(Name = "scene_id")] Guid sceneId) =>
+        Ok(await svc.GetContentElementsAsync(sceneId));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPost]
+    public async Task<IActionResult> Create([FromBody] SaveContentElementRequest req) =>
+        Ok(await svc.CreateContentElementAsync(req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpPut("{id:guid}")]
+    public async Task<IActionResult> Update(Guid id, [FromBody] SaveContentElementRequest req) =>
+        Ok(await svc.UpdateContentElementAsync(id, req));
+
+    [Authorize(Roles = "Admin")]
+    [HttpDelete("{id:guid}")]
+    public async Task<IActionResult> Delete(Guid id)
+    {
+        await svc.DeleteContentElementAsync(id);
+        return NoContent();
+    }
+}
+
 [ApiController]
 [Route("v1/shared-colors")]
 public class SharedColorsController(SceneColorService svc) : ControllerBase