AO3-7045 Migrate audits table to BIGINT

[alyssapowell]

Issue

https://otwarchive.atlassian.net/browse/AO3-7045

Purpose

Migrates the audits table primary key from integer to BIGINT to prevent integer overflow as the audit log grows. This is a preventive infrastructure improvement to handle the increasing volume of audit records in the system.

Testing Instructions

1. Run the migration in a development environment: rails db:migrate
2. Verify the schema change: Check that audits.id column is now BIGINT type
3. Test audit functionality: Ensure audit records are still created for tracked changes
4. Run existing test suite to verify no functionality breaks

References

This follows the same pattern as recent similar migrations:

• 20250704060231_change_bookmark_foreign_keys_type.rb
• 20250718220533_change_subscriptions_id_to_bigint.rb

Credit

Alyssa Powell (she/her)