Delete a room's messages in a job, one transaction each

Room#destroy destroyed every message inside the room's own
transaction, which holds SQLite's write lock until the last one: on a
room with many messages, every other write in the app waited and
failed. The request now takes the room away from its members and
leaves the rest to Room::DestroyJob, which destroys the messages one
at a time, each in its own short transaction, and then the room.

An open room is closed in the request, so that someone who joins the
account before the job ends isn't given it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Bj8KnxpTf9sj2Ysa8aLAVa

app/controllers/rooms_controller.rb

@@ -12,7 +12,7 @@ class RoomsController < ApplicationController
   end
 
   def destroy
-    @room.destroy
+    @room.destroy_later
 
     broadcast_remove_room
     redirect_to root_url