AO3-7047 Migrate comments table to BIGINT

[alyssapowell]

Issue

https://otwarchive.atlassian.net/browse/AO3-7047

Purpose

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

Testing Instructions

1. Run the migration in a development environment: rails db:migrate
2. Verify the schema change: Check that comments.id column is now BIGINT type
3. Test comment functionality: Ensure comments can still be created, edited, and displayed
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)

[Bilka2]

Hi!

Thank you so much for this pull request. For reviewing this, I am taking the liberty to address all your currently open BIGINT PRs here with one comment since the same applies to all of them. As described on Jira, these migrations also need to be done for all columns that refer to model ID that's getting migrated. Furthermore, please fix the rubocop warnings - we generally try to make new or changed code conform to our linters. For these big int PRs we'd like to have regression tests in the form of specs, you can refer to #5278 or #5294 for examples. For models with foreign keys in other tables please also test those, similar to what's done for bookmarks.

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