fix: default AIRFLOW_UID to 50000 in airflow-init chown lines

[JaeHyuckSa]

airflow-init's chown/echo lines use ${AIRFLOW_UID} without a default. If. the host doesn't set AIRFLOW_UID, compose substitutes it to "" at parse time and the script ends up running chown -R ":0" /opt/airflow/. Ownership stays unchanged, only the group flips to 0. Workers run as 50000 (per the user: line, which already uses ${AIRFLOW_UID:-50000}), can't write to logs, heartbeats fail, DAGs go zombie.

Add the same :-50000 default to the four chown/echo lines.

#50162 tried this earlier with ${CHOWN_UID}, also without a default.

closes: #48931

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Was generative AI tooling used to co-author this PR?

• Yes — Cursor Composer 2.0

Generated-by: Cursor Composer 2.0 following the guidelines

cc. @choo121600

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[boring-cyborg]

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