DEV: Use Single Table Inheritance in TopicTimer (#34530)

To prepare for the introduction of `CategoryDefaultTimer`, which allows
posts created in certain categories to have a default `TopicTimer`, we
first need to convert `TopicTimer` to use [Single Table
Inheritance][STI].

[STI]:
https://guides.rubyonrails.org/association_basics.html#single-table-inheritance-sti

db/migrate/20250826012802_add_type_to_topic_timer.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+class AddTypeToTopicTimer < ActiveRecord::Migration[8.0]
+  def change
+    add_column :topic_timers, :type, :string, null: false, default: "TopicTimer"
+    remove_index :topic_timers, name: :idx_topic_id_public_type_deleted_at
+    add_index :topic_timers,
+              :topic_id,
+              unique: true,
+              name: :idx_topic_id_public_type_deleted_at,
+              where: "public_type = true AND deleted_at IS NULL AND type = 'TopicTimer'"
+  end
+end