feat: add opt-in durable library indexing requests

[shatrughnaupadhyay]

Adds an opt-in database outbox for library create/rename indexing, post-commit Celery dispatch, bounded document batches and a recovery command. Duplicate deliveries coalesce; failures leave work pending. The feature defaults to disabled.

20 focused CMS tests passed. Disposable MySQL checks passed migration apply/rollback, concurrent requests, duplicate workers and interrupted-task replay. A real Redis outage/recovery check passed with the engine boundary mocked. Ruff, configured Pylint and migration consistency passed.

The worker holds a library row lock during indexing. Per-item writes remain outside that lock; comprehensive stale-write prevention and frontend pending-state handling need further work before activation.

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