DEV: Add messages_count to ChatChannel table (#19295)

This commit adds the messages_count column for ChatChannel messages,
which is the number of not-deleted messages in the channel.

This is not updated every time a message is created or deleted in a
channel, so it should not be displayed in the UI.
It is updated eventually via Jobs::ChatPeriodicalUpdates, which
will have additional functions in future after being introduced
here.

Also update these counts for existing channels in a post migration.

plugins/chat/app/jobs/scheduled/chat_periodical_updates.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module Jobs
+  class ChatPeriodicalUpdates < ::Jobs::Scheduled
+    every 15.minutes
+
+    def execute(args = nil)
+      # TODO: Add rebaking of old messages (baked_version <
+      # ChatMessage::BAKED_VERSION or baked_version IS NULL)
+      ChatChannel.ensure_consistency!
+      nil
+    end
+  end
+end

plugins/chat/app/jobs/scheduled/update_user_counts_for_chat_channels.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 module Jobs
+  # TODO (martin) Move into ChatChannel.ensure_consistency! so it
+  # is run with ChatPeriodicalUpdates
   class UpdateUserCountsForChatChannels < ::Jobs::Scheduled
     every 1.hour