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/gateway/cmd/server/main.go

@@ -118,6 +118,7 @@ func main() {
 	v1.Any("/favorites/*path",    apiRL, auth, content.Handler())
 	v1.Any("/ai/*path",           apiRL, auth, content.Handler())
 	v1.Any("/scenes/*path",       apiRL, optionalAuth, content.Handler())
+	v1.Any("/scene-elements/*path", apiRL, optionalAuth, content.Handler())
 	v1.Any("/shared-colors/*path", apiRL, optionalAuth, content.Handler())
 	v1.Any("/color-presets/*path", apiRL, optionalAuth, content.Handler())