chore: temporarily disable ProcessStaleContactsJob (#13462)

We've been seeing orphan conversations (conversations whose contact has
been deleted) on high-frequency accounts. These orphans cause 500 errors
when the API attempts to render conversation data, since the jbuilder
partials expect a valid contact association.

The `ProcessStaleContactsJob` triggers `RemoveStaleContactsService`,
which uses `delete_all` to remove contacts. While the query only selects
contacts without conversations, `delete_all` bypasses ActiveRecord
callbacks and does not re-verify at deletion time. We suspect this
creates a window where a new conversation can be created for a contact
between query evaluation and the actual delete, leaving the conversation
orphaned.

**This is unverified — disabling the job is the experiment to confirm or
rule out this theory.**

This PR disables the job for a monitoring period. If orphan
conversations stop appearing, we'll have confirmation and can work on a
proper fix for the service. If they continue, we'll investigate other
deletion paths.

Stale contacts will accumulate while the job is disabled, but this is a
controlled tradeoff — they are inert records with no user-facing impact,
and can be cleaned up in bulk once we re-enable or fix the job.

config/schedule.yml

@@ -33,12 +33,13 @@ remove_stale_redis_keys_job.rb:
   class: 'Internal::RemoveStaleRedisKeysJob'
   queue: scheduled_jobs
 
-#executed daily at 0430 UTC
-# which will be IST 10:00 AM
-process_stale_contacts_job:
-  cron: '30 04 * * *'
-  class: 'Internal::ProcessStaleContactsJob'
-  queue: housekeeping
+# DISABLED: investigating if this job is the source of orphan conversations
+# #executed daily at 0430 UTC
+# # which will be IST 10:00 AM
+# process_stale_contacts_job:
+#   cron: '30 04 * * *'
+#   class: 'Internal::ProcessStaleContactsJob'
+#   queue: housekeeping
 
 # executed daily at 0100 UTC
 # to delete accounts marked for deletion