Epic: Background Jobs (uptime, webhooks, email, alerts)

Make the configured-but-dormant features actually execute on a schedule.

Tracks (4):

• #90 — Jobs: a scheduled-jobs runtime (cron/Lambda schedule) for all periodic work
• #91 — Jobs: uptime monitors actually probe URLs + record status/history
• #92 — Jobs: webhooks fire on events (delivery + retries + signing)
• #93 — Jobs: alert evaluation runs on schedule + dispatches notifications

Part of the Fathom + Sentry product backlog. See #44 for the original roadmap.

Epic complete — closed retroactively. Scheduler runtime (#90) + all four feature jobs shipped and live-verified via /api/jobs/tick: error-alert evaluation + notify (#80/#93), uptime probing (#91), signed webhook delivery with retries (#92), email digests (#89). The runtime also now drives the #94 daily rollups.