Allow bots to update and destroy their own messages

Bots can only create. A lifecycle notification — an alert that fires and then
resolves, a deploy that starts and finishes, a backup that runs — therefore has
to post a second message, and the room becomes an append-only log of states
rather than a view of the current one.

Adds PATCH and DELETE inside the existing bot_key scope, routed to
Messages::ByBotsController. The body is read the way create reads it, so
updating a message is the same request shape as posting one.

No new authorization: both actions already run through ensure_can_administer,
and can_administer? grants access only to a record the user created, so a bot
key reaches that bot's own messages and no others. set_room narrows it again by
looking the room up through the bot's own memberships. A leaked bot key gains
what it could already do by posting: write to rooms that bot belongs to.

update answers head :ok rather than the redirect, which meant extracting the
update and its broadcast into update_message — calling super and then head
would double render, since the parent redirects inside the action. destroy
needs no split, because the parent renders implicitly like create does.

app/controllers/messages_controller.rb

@@ -34,9 +34,7 @@ class MessagesController < ApplicationController
   end
 
   def update
-    @message.update!(message_params)
-
-    @message.broadcast_replace_to @room, :messages, target: [ @message, :presentation ], partial: "messages/presentation", attributes: { maintain_scroll: true }
+    update_message
     redirect_to room_message_url(@room, @message)
   end
 
@@ -50,6 +48,13 @@ class MessagesController < ApplicationController
       @message = @room.messages.find(params[:id])
     end
 
+    # Extracted so bots can reuse the update and its broadcast while answering with
+    # a status code instead of a redirect.
+    def update_message
+      @message.update!(message_params)
+      @message.broadcast_replace_to @room, :messages, target: [ @message, :presentation ], partial: "messages/presentation", attributes: { maintain_scroll: true }
+    end
+
     def ensure_can_administer
       head :forbidden unless Current.user.can_administer?(@message)
     end