Add the organization_id column to pool_repositories

What does this MR do and why?

• adds organization_id to pool_repositories with null: true
• updates gitlab/spec/db/schema_spec.rb foreign key constraint list
  • the foreign key will be handled in a backfill migration that will come after this

Future MRs will add code support, backfill migrations, NOT NULL constraint, and model validations

migration output

be rake db:migrate
main: == [advisory_lock_connection] object_id: 155460, pg_backend_pid: 75395
main: == 20250929180950 AddOrganizationIdToPoolRepositories: migrating ==============
main: -- add_column(:pool_repositories, :organization_id, :bigint, {:null=>true})
main:    -> 0.0404s
main: == 20250929180950 AddOrganizationIdToPoolRepositories: migrated (0.0457s) =====

main: == [advisory_lock_connection] object_id: 155460, pg_backend_pid: 75395
ci: == [advisory_lock_connection] object_id: 155460, pg_backend_pid: 75423
ci: == 20250929180950 AddOrganizationIdToPoolRepositories: migrating ==============
ci: -- add_column(:pool_repositories, :organization_id, :bigint, {:null=>true})
ci:    -> 0.0050s
ci: == 20250929180950 AddOrganizationIdToPoolRepositories: migrated (0.0133s) =====

ci: == [advisory_lock_connection] object_id: 155460, pg_backend_pid: 75423

Related to #519615 (closed)