[CPDLP-3952] Add index to participant declaration type field

[leandroalemao]

Context

• Ticket: CPDLP-3952

We don’t have an index on the type on participant declarations.

Changes proposed in this pull request

• Add an index to the participant declaration field type

Snapshot db:

== 20250124134039 AddTypeIndexToParticipantDeclarations: migrating ============
-- add_index(:participant_declarations, :type, {:algorithm=>:concurrently})
   -> 0.7996s
== 20250124134039 AddTypeIndexToParticipantDeclarations: migrated (1.2468s) ===

[github-actions]

Review app deployed to https://cpd-ecf-review-5475-web.test.teacherservices.cloud