Cron job exits non-zero when queue is empty

[rdyson]

Problem

When the daily queue is empty, generate_digest.py exits with status code 1. Because run_daily.sh uses set -e, the entire cron run aborts.

Repro

1. Ensure data/queue/articles.json does not exist (or is empty).
2. Run ./run_daily.sh.

Expected

The daily workflow should treat "no queued articles" as a normal condition and exit successfully, still running any subsequent maintenance steps.

Actual

generate_digest.py prints "No digest generated." and exits non-zero, which aborts run_daily.sh.

Impact

• No-op days appear as failures in cron.
• Cleanup step may be skipped.
• Monitoring/alerting gets noisy.

References

• generate_digest.py:159
• generate_digest.py:161
• run_daily.sh:13
• run_daily.sh:32