DEV: consolidate chat channel notification settings (#29080)

On the chat channel settings page, we want to show a single Send push notifications setting instead of the current Desktop notifications and Mobile push notifications settings.

For existing users, use the Mobile push notifications setting value for the new Send push notifications setting.

plugins/chat/db/migrate/20241003122030_add_notification_level_to_user_chat_channel_memberships.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+class AddNotificationLevelToUserChatChannelMemberships < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def up
+    execute <<~SQL
+      DROP INDEX CONCURRENTLY IF EXISTS user_chat_channel_memberships_index
+    SQL
+
+    add_column :user_chat_channel_memberships, :notification_level, :integer, null: true
+
+    batch_size = 10_000
+    min_id, max_id = execute("SELECT MIN(id), MAX(id) FROM user_chat_channel_memberships")[0].values
+
+    (min_id..max_id).step(batch_size) { |start_id| execute <<~SQL.squish } if min_id && max_id
+      UPDATE user_chat_channel_memberships
+      SET notification_level = mobile_notification_level
+      WHERE id >= #{start_id} AND id < #{start_id + batch_size}
+    SQL
+
+    change_column_default :user_chat_channel_memberships, :notification_level, 1
+    change_column_null :user_chat_channel_memberships, :notification_level, false
+
+    execute <<~SQL
+      CREATE INDEX CONCURRENTLY user_chat_channel_memberships_index ON user_chat_channel_memberships using btree (user_id, chat_channel_id, notification_level, following)
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end