feat: Run assignment every 15 minutes (#12334)

Currently, auto-assignment runs only during conversation creation or
update events. If no agents are online when new conversations arrive,
those conversations remain unassigned.

With this change, unassigned conversations will be automatically
assigned once agents become available. The job runs every 15 minutes and
uses a fair distribution threshold of 100 to prevent a large number of
conversations from being assigned to a single available agent. This will
be customizable later.

lib/limits.rb

@@ -5,6 +5,7 @@ module Limits
   OUT_OF_OFFICE_MESSAGE_MAX_LENGTH = 10_000
   GREETING_MESSAGE_MAX_LENGTH = 10_000
   CATEGORIES_PER_PAGE = 1000
+  AUTO_ASSIGNMENT_BULK_LIMIT = 100
 
   def self.conversation_message_per_minute_limit
     ENV.fetch('CONVERSATION_MESSAGE_PER_MINUTE_LIMIT', '200').to_i