Respect configured cron timezone

[fallintoplace]

Summary

Make cron scheduling honor the configured Nerve timezone instead of the server/container timezone.

Root cause

NerveConfig.timezone was defined and documented, but CronService created APScheduler and cron triggers without passing it through. APScheduler therefore defaulted to the host timezone. context_rotate_at had the same problem because it derived "local" time from the OS timezone.

Changes

• Create a single ZoneInfo(config.timezone) for cron service scheduling.
• Pass that timezone to the scheduler, cron triggers, interval triggers, cleanup, wakeup sweep, and source schedules.
• Use the configured timezone for catch-up crontab checks and daily context rotation times.
• Add regression tests for scheduler timezone, trigger timezone, catch-up timezone, and context_rotate_at.

Validation

• python -m pytest tests/test_cron.py -q