Trigger-based monitor schedules can now run trigger-only

Monitors on a dynamic schedule — "when the table updates" or "when a job/task completes" — previously always forced a fallback run if no update or job completion occurred within the fallbacks set window. That fallback is now optional.

"Run periodically if trigger doesn't fire" stays on by default but can be disabled ( UI, API, and monitors-as-code (set interval_minutes: 0)).