FIX: more performance improvement for PostAlert job (#22487)

Simplified query based on SiteSettings to join only relevant user_options rows.
In addition, index was added to 'watched_precedence_over_muted` column in `user_options` table to speed up query

db/migrate/20230707031122_add_watched_precedence_over_muted_index_to_user_options.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddWatchedPrecedenceOverMutedIndexToUserOptions < ActiveRecord::Migration[7.0]
+  def change
+    add_index :user_options, :watched_precedence_over_muted
+  end
+end

db/migrate/20230712011946_add_topic_id_notification_level_index_to_topic_users.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+#
+class AddTopicIdNotificationLevelIndexToTopicUsers < ActiveRecord::Migration[7.0]
+  def change
+    add_index :topic_users, %i[topic_id notification_level]
+  end
+end

db/migrate/20230712013248_add_category_id_notification_level_index_to_category_users.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddCategoryIdNotificationLevelIndexToCategoryUsers < ActiveRecord::Migration[7.0]
+  def change
+    add_index :category_users, %i[category_id notification_level]
+  end
+end