feat(logging): add datadog context, celery structure logs

[JacobCoffee]

Split off from https://github.com/pypi/warehouse/pull/18748

worker-1        | {"event": "beat: Acquired lock", "timestamp": "2025-10-10T20:42:30.035157Z", "level": "info", "dd.env": "development", "dd.version": "unknown"}
worker-1        | {"event": "mingle: all alone", "timestamp": "2025-10-10T20:42:31.034605Z", "level": "info", "dd.env": "development", "dd.version": "unknown"}
worker-1        | {"event": "celery@7e55216e877e ready.", "timestamp": "2025-10-10T20:42:31.043095Z", "level": "info", "dd.env": "development", "dd.version": "unknown"}

structures celery logs, injects dd context.

based on https://www.structlog.org/en/stable/frameworks.html#celery

pairs with https://github.com/pypi/warehouse/pull/18844