Adds indexes on service_desk_custom_email_verifications

What does this MR do and why?

For the Service Desk custom email feature, where we have email verification, we'll need a recurring task that closes all started and overdue verifications. It will use a query like:

# Something along these lines
ServiceDesk::CustomEmailVerification.started.overdue
# translates to something like
ServiceDesk::CustomEmailVerification.where(state: :started).where('triggered_at <= ?', 30.minutes.ago)

That produces a query like:

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')

This scheduled job will run every few minutes.

This MR adds indexes to the state and triggered_at fields of service_desk_custom_email_verifications to lay out the foundation for the cleanup task.

Query plans

Because the feature is still behind a feature flag, there are now rows to select in postgres.ai.

Using example query from above 👆🏻

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 without indexes

Seq Scan on service_desk_custom_email_verifications  (cost=0.00..49.00 rows=1 width=108)  
  Filter: ((triggered_at <= '2023-08-04 13:21:28.666463+00'::timestamp with time zone) AND (state = 0)) 

Plan with two indexes (a37c8568)

Index Scan using index_service_desk_custom_email_verifications_on_state on service_desk_custom_email_verifications  (cost=0.15..6.21 rows=1 width=108)  
  Index Cond: (state = 0)  
  Filter: (triggered_at <= '2023-08-04 13:21:28.666463+00'::timestamp with time zone)

Executed this statement before the plan:

CREATE INDEX index_service_desk_custom_email_verifications_on_state ON service_desk_custom_email_verifications (state);
CREATE INDEX index_service_desk_custom_email_verifications_on_triggered_at ON service_desk_custom_email_verifications (triggered_at);

Plan with one partial index (current version 2fa95847)

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)  

Executed this statement before the plan:

CREATE INDEX i_custom_email_verifications_on_triggered_at_and_state_started ON 
i_custom_email_verifications_on_triggered_at_and_state_started (triggered_at)
WHERE state = 0;

Screenshots or screen recordings

🚫 migration only

How to set up and validate locally

🚫 migration only

MR acceptance checklist

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

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