Fix: Overlapping Cron Jobs Running Simultaneously

When a cron job takes longer than its schedule interval, the next invocation starts before the previous one finishes. Both run simultaneously, competing for resources and potentially corrupting data.

# Before (unsafe):
*/5 * * * * /usr/local/bin/sync.sh

# After (flock-safe):
*/5 * * * * flock -n /tmp/sync.lock /usr/local/bin/sync.sh

flock -n exits immediately if the lock is already held. The kernel releases the lock automatically when the process exits — even on crash.

Detect current overlaps

ps aux | grep sync.sh | grep -v grep
# Multiple lines = overlapping runs