feat: configurable check-in margin and max runtime for scheduled task monitors

[stefanpoensgen]

Fixes #19

ScheduledTaskSubscriber upserts monitors with only a schedule, so Sentry expects the check-in exactly at the scheduled minute. The task runner fires a few seconds after that minute, and Sentry reports the run as missed. In our project this produced about one false "Cron failure" issue per day for an hourly task that ran fine every time.

This adds two optional settings that are passed through to MonitorConfig:

frosh_sentry:
    report_scheduled_tasks: true
    scheduled_tasks:
        checkin_margin: 5
        max_runtime: 30

Both default to null, so existing setups keep Sentry's monitor defaults.

https://claude.ai/code/session_01G2xLjqD8vZ7Ys3inHx4ax5

[stefanpoensgen]

The failing phpstan job is the pre-existing failure on main (PHPStan 1.x vs. the symfony/config generics); #44 fixes it. Happy to rebase once that is in.