Trim old task logs in the Docker Compose quick-start

[Admaing]

The quick-start kept every task log forever, so the shared logs volume grows without bound until the
disk of the machine running it fills up.

The Helm chart has had a logGroomerSidecar for a long time, but the docker-compose.yaml that most
people start with had no equivalent, and the logging docs never said that Airflow neither rotates nor
deletes local task logs.

This adds an airflow-log-groomer service that reuses the same /clean-logs script as the chart:

• task logs older than 15 days are deleted by default (the same default as the Helm chart), configurable
  with AIRFLOW__LOG_RETENTION_DAYS / AIRFLOW__LOG_RETENTION_MINUTES;
• AIRFLOW__LOG_MAX_SIZE_BYTES / AIRFLOW__LOG_MAX_SIZE_PERCENT bound the size of the logs folder;
• removing the airflow-log-groomer service keeps all logs, as before;
• the behaviour is documented in the quick-start page, in "Logging for Tasks", and in the advanced
  logging configuration page, which previously only mentioned size-based rotation.

Note for reviewers: the AIRFLOW__LOG_* variables are read by the /clean-logs script, not by Airflow
itself. Setting them in .env therefore adds them to the environment of every service, where Airflow
ignores them (AIRFLOW__LOG_RETENTION_DAYS has no __ separator between a section and a key, so it is
not a configuration option), while the groomer picks them up through Compose interpolation. The names
are the ones the chart and the script already use.

Verified locally: docker compose config renders the service for both the default and an overridden
retention, and yamllint, the Compose JSON-schema hook, mypy for docker-tests and the remaining prek
hooks pass for the changed files. The new test_log_groomer_service_config asserts the service, its
command and the retention default/override against that same rendered config; it runs as part of the
docker-compose-tests suite, which was not run locally.

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DeepSeek Harness (deepseek-v4.1-flash) following the guidelines

[boring-cyborg]

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