PERF: Cache Category.subcategory_ids (#9350)

Also reset category cache after backup restore.

lib/backup_restore/restorer.rb

@@ -52,6 +52,7 @@ module BackupRestore
 
       @system.disable_readonly_mode
 
+      clear_category_cache
       clear_emoji_cache
       clear_theme_cache
 
@@ -119,6 +120,12 @@ module BackupRestore
       end
     end
 
+    def clear_category_cache
+      log "Clearing category cache..."
+      Category.reset_topic_ids_cache
+      Category.clear_subcategory_ids
+    end
+
     def clear_emoji_cache
       log "Clearing emoji cache..."
       Emoji.clear_cache