FIX: Allow `category` to be set on `topic` model instances (#25416)

Some parts of our code (e.g. some types of reviewable) set `topic.category`. In the past this would override the computed property value, but with recent Ember it raises an error. This commit adds a setter which handles the situation cleanly

app/assets/javascripts/discourse/app/models/topic.js

@@ -1,4 +1,4 @@
-import EmberObject from "@ember/object";
+import EmberObject, { computed } from "@ember/object";
 import { alias, and, equal, notEmpty, or } from "@ember/object/computed";
 import { Promise } from "rsvp";
 import { resolveShareUrl } from "discourse/helpers/share-url";
@@ -214,11 +214,15 @@ const Topic = RestModel.extend({
     return { type: "topic", id };
   },
 
-  @discourseComputed("category_id")
-  category() {
+  @computed("category_id")
+  get category() {
     return Category.findById(this.category_id);
   },
 
+  set category(newCategory) {
+    this.set("category_id", newCategory?.id);
+  },
+
   @discourseComputed("url")
   shareUrl(url) {
     const user = User.current();