fix(schema.rb): remove unbacked columns added by self-check-in migrations

[mroderick]

What

Removes check_in_code and source columns/indexes from db/schema.rb that are not backed by any migration file on master.

Why

The only migrations that add these columns live on feature/self-check-in (commit 1ed1a544), which has not been merged to master. Having them in db/schema.rb without corresponding migrations makes the schema inconsistent with db/migrate and breaks the ability to replay migrations from scratch.

This is a prerequisite for the migration-squash work in feature/squash-pre-2026-migrations.

Verification

• ruby -c db/schema.rb passes.
• Hiding db/schema.rb and running RAILS_ENV=test TEST_ENV_NUMBER=fixverify bundle exec rails db:create db:migrate reproduces the committed db/schema.rb exactly (diff is empty).

Removed

• events.check_in_code + unique index
• workshops.check_in_code + unique index
• invitations.source
• workshop_invitations.source