[19.0] base_import_async: migrate

[tishmen]

Scope

• base_import_async only

Depends

• Base PR: [19.0] queue_job: migrate + tests #834

Summary

• Migrate manifest and code to 19.0; keep assets relative.

Pre-commit

• Ran; committed auto-fixes where needed.

Tests

• Local environment lacks core module base_import, so install is skipped; CI on OCA infra will validate with full core present.

[tishmen]

Closing as requested; will re-open after adjustments.