Add experimental server-side notification grouping (#29889)

db/migrate/20240513095755_add_group_key_to_notifications.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddGroupKeyToNotifications < ActiveRecord::Migration[7.1]
+  def change
+    add_column :notifications, :group_key, :string
+  end
+end

db/migrate/20240513123807_add_index_notifications_on_account_id_and_group_key.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class AddIndexNotificationsOnAccountIdAndGroupKey < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :notifications, [:account_id, :group_key], algorithm: :concurrently, where: 'group_key IS NOT NULL'
+  end
+end