Adds Service Desk custom email cleanup worker

What does this MR do and why?

This MR adds a cronjob worker ServiceDesk::CustomEmailCleanupWorker that marks started and overdue verifications for Service Desk custom email addresses as failed.

Verification workflow: We send a verification email to the custom email which should get forwarded to the Service Desk email address of the project and we then ingest that email. This should usually take only a couple of minutes. If we didn't receive anything within the last 30 minutes, we need to end the verification process for that project, mark the verification as failed and send the verification result emails. This is what the worker will do.

The worker will run every two minutes.

🎏 This feature is hidden behind a feature flag and not used on production yet.

Screenshots or screen recordings

🚫 backend only

Query

The DB index was added with Adds indexes on service_desk_custom_email_verif... (!128498 - merged)

Query

SELECT "service_desk_custom_email_verifications".* 
FROM "service_desk_custom_email_verifications" 
WHERE "service_desk_custom_email_verifications"."state" = 0 AND 
(triggered_at <= '2023-08-04 13:21:28.666463')

Plan

Index Scan using i_custom_email_verifications_on_triggered_at_and_state_started on service_desk_custom_email_verifications  (cost=0.12..3.14 rows=1 width=108)  
  Index Cond: (triggered_at <= '2023-08-04 13:21:28.666463+00'::timestamp with time zone)  

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open the rails console and add records for a custom email address for a project that is overdue:

   project = Project.find(7)
   current_user = User.first
   custom_email = 'support@example.com'
   
   setting = ServiceDeskSetting.find_or_create_by!(project_id: project.id)
   setting.update!(custom_email: custom_email)
   
   credential = ServiceDesk::CustomEmailCredential.create!(
     project_id: project.id,
     smtp_address: 'smtp.gmail.com', # we need a valid smtp address
     smtp_port: 587,
     smtp_username: custom_email,
     smtp_password: 'supersecret'
   )
   
   verification = ServiceDesk::CustomEmailVerification.new(
     project_id: project.id
   )
   verification.mark_as_started!(current_user)
   verification.update!(triggered_at: 31.minutes.ago)
   
   ServiceDesk::CustomEmailVerificationCleanupWorker.new.perform
   
   verification.reset

2. The verification print should say something like

   state: "failed",
   error: "mail_not_received_within_timeframe",

3. You can also leave out the call of the worker and wait until the cronjob runs and then reset the verification object to see that it changed the state.

4. In letter opener (if you have no other email config) you should see a verification result email to User.first with the error.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.