PERF: Optimize Category.subcategories (#31802)

At the moment, every call to `Category.subcategories` causes an
iteration through every single category. For sites with thousands of
categories, this is incredibly expensive, and makes things like the
category dropdown & sidebar rendering very slow.

To improve things, we can create a map of parent_category => [child
categories] in a single iteration, cache it, and then use that when
finding subcategories.

The `@computed` decorator ensures that this cache will be recalculated
whenever the list of categories changes (e.g. when sideloading new
categories in the experimental lazy-loaded-categories mode)

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

@@ -503,9 +503,9 @@ export default class Category extends RestModel {
     this.set("parent_category_id", newParentCategory?.id);
   }
 
-  @computed("site.categories.[]")
+  @computed("site.categoriesByParentId")
   get subcategories() {
-    return this.site.categories.filterBy("parent_category_id", this.id);
+    return this.site.categoriesByParentId.get(this.id) || [];
   }
 
   @computed("subcategories")

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

@@ -147,6 +147,17 @@ export default class Site extends RestModel {
     return map;
   }
 
+  @computed("categories.@each.parent_category_id")
+  get categoriesByParentId() {
+    const map = new Map();
+    for (const category of this.categories) {
+      const siblings = map.get(category.parent_category_id) || [];
+      siblings.push(category);
+      map.set(category.parent_category_id, siblings);
+    }
+    return map;
+  }
+
   @discourseComputed("notification_types")
   notificationLookup(notificationTypes) {
     const result = [];