Background migration: Update step_url from company path

What does this MR do and why?

Adds a post-deployment background migration to update user_details.onboarding_status.step_url values from either /users/sign_up/company or /users/sign_up/groups/new to /users/sign_up/welcome.

Relates to https://gitlab.com/gitlab-org/gitlab/-/work_items/593994.

Files added:

• lib/gitlab/background_migration/update_step_url_to_welcome_path.rb — BBM class that updates matching step_url values in batches
• db/post_migrate/20260715142246_queue_update_step_url_to_welcome_path.rb — post-deployment migration that queues the BBM
• db/docs/batched_background_migrations/update_step_url_to_welcome_path.yml — BBM dictionary entry
• spec/lib/gitlab/background_migration/update_step_url_to_welcome_path_spec.rb — BBM spec
• spec/migrations/20260715142246_queue_update_step_url_to_welcome_path_spec.rb — queue migration spec

Query related

Estimated 2.7 million rows to update: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/52139/commands/153587.

Example update query:

UPDATE "user_details"
  SET "onboarding_status" = jsonb_set(
    onboarding_status,
    '{step_url}',
    '"/users/sign_up/welcome"'::jsonb
  )
  WHERE "user_details"."user_id" >= 1
    AND "user_details"."user_id" < 251
    AND (onboarding_status ->> 'step_url' IN ('/users/sign_up/company', '/users/sign_up/groups/new'));

Query plan: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/52139/commands/153590

How to set up and validate locally

1. In a rails console, create a user with step_url set to one of the old paths:

   user = User.last
   user.update!(onboarding_status_step_url: '/users/sign_up/company')

2. Run the migration:

   Gitlab::BackgroundMigration::UpdateStepUrlToWelcomePath.new(
     start_id: user.user_detail.user_id,
     end_id: user.user_detail.user_id,
     batch_table: :user_details,
     batch_column: :user_id,
     sub_batch_size: 100,
     pause_ms: 0,
     connection: ApplicationRecord.connection
   ).perform

3. Verify the step_url was updated:

   user.reload.onboarding_status_step_url
   # => "/users/sign_up/welcome"

MR acceptance checklist

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