Cap bulk REST operations and add the WP-CLI escape hatch

[priethor]

Goal

Bulk REST operations run synchronously today. Large collections can time out or run out of memory. Cap REST requests and send larger jobs through WP-CLI instead.

Scope

• Cap REST bulk operations at 1000 affected rows.
• For migrate_rows, count rows that hold the source value.
• For delete_dependent_rollups_for_field, count rows across every rollup that depends on the deleted field.
• Over the cap, return HTTP 400 with cortext_bulk_op_requires_cli and a message containing the exact CLI command to run.
• Add uncapped WP-CLI commands with progress and resume:
  • wp cortext migrate-field <id> --from=<value> --to=<value>
  • wp cortext rebuild-dependent-rollups
• Add Cortext\Background\BulkRunner with a sync implementation so an async runner can be added later without changing callers.

Acceptance criteria

• FieldsController::migrate_rows and delete_dependent_rollups_for_field return 400 with the CLI command when the request is above the cap.
• Both CLI commands complete on a large dataset with a progress bar.
• Under-cap behavior is unchanged.