add-or-update async Celery flow

[whisper67265]

Problem

The async add-or-update endpoint returns a 202 with a task_id, but no integration test verifies the full flow including Celery task completion.

Acceptance Criteria

• Valid POST /boost-endpoint/add-or-update/ returns 202 with task_id
• Celery task completes successfully
• Invalid request returns 400; unauthenticated returns 401
• Test runs as part of integration-functional.yml
• PR approved by at least 1 reviewer

References

• Related files: boost_weblate/endpoint/views.py, boost_weblate/endpoint/tasks.py