Add timeline read markers API (#11762)

Fix #4093

app/models/marker.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: markers
+#
+#  id           :bigint(8)        not null, primary key
+#  user_id      :bigint(8)
+#  timeline     :string           default(""), not null
+#  last_read_id :bigint(8)        default(0), not null
+#  lock_version :integer          default(0), not null
+#  created_at   :datetime         not null
+#  updated_at   :datetime         not null
+#
+
+class Marker < ApplicationRecord
+  TIMELINES = %w(home notifications).freeze
+
+  belongs_to :user
+
+  validates :timeline, :last_read_id, presence: true
+  validates :timeline, inclusion: { in: TIMELINES }
+end