Add explicity primary key definition to AuditEvent

As a part of the change for partitioning audit_events, the AuditEvent
model will need to have the primary_key explicitly set to use only the
id column. While the database table also includes the partition key
(created_at) in the primary key definition, due to Rails support for
multi-column PK it's easier to simply use the id column, which should
still be unique as it's backed by a sequence.

app/models/audit_event.rb

@@ -13,6 +13,8 @@ class AuditEvent < ApplicationRecord
     :target_id
   ].freeze
 
+  self.primary_key = :id
+
   serialize :details, Hash # rubocop:disable Cop/ActiveRecordSerialize
 
   belongs_to :user, foreign_key: :author_id