Add organization_id to bulk_import_export_batches

What does this MR do and why?

Part 1 of 2: Add organization_id column to bulk_import_export_batches

Adds organization_id to bulk_import_export_batches (step 2/3 of #603592 (closed); parent table done in #600457 (closed) via !241088 (merged) / !241089 (merged)). Schema-only, no data changes:

• Adds nullable organization_id (bigint)
• Adds a concurrent index on it (idx_bulk_import_export_batches_on_organization_id)
• Adds a FK to organizations (on_delete: :cascade, NOT VALID; validation comes in the finalize phase, #603593 (closed))
• Adds desired_sharding_key_migration_job_name to db/docs/bulk_import_export_batches.yml and allowlists the table in spec/lib/gitlab/organizations/sharding_key_spec.rb

The backfill (trigger + BBM) follows in Part 2.

Note for reviewers: this is a stacked MR. Please review Part 2 as well: !243559 (closed).

Table size: table_size: small per db/docs, ~808K rows on GitLab.com — well under the 50GB FK-addition limit.

Migration details

db/migrate/20260702120000_add_organization_id_to_bulk_import_export_batches.rb
db/post_migrate/20260702120001_add_concurrent_index_organization_id_on_bulk_import_export_batches.rb
db/post_migrate/20260702120002_add_foreign_key_constraint_on_bulk_import_export_batches_organization_id.rb

MR acceptance checklist

• Migrations tested up and down locally
• Database review

References

• Issue #603592 (closed) (parent #600457 (closed) done; finalize is #603593 (closed) and out of scope here)
• Mirrors merged !241088 (merged) for the parent table bulk_import_exports