[ajianaz]

What

• Sentry Crons compatibility: check_in envelope items are parsed and stored; monitors auto-register on first check-in (monitors + monitor_checkins, migration 007 Postgres / timestamped SQLite).
• Hourly staleness evaluation: a crontab-scheduled monitor that misses its expected check-in (+ checkin_margin, default 60 min) flips to missed and emits an alert. UTC-only 5-field crontab matcher, no new dependencies.
• Alert engine generalized to AlertSignal::{Issue, MonitorMissed} — one webhook pipeline for both signal types, existing cooldown respected. Monitor-missed webhook payload: {"type":"monitor_missed",...}.
• APIs: GET /api/0/projects/{slug}/monitors, GET /api/0/projects/{slug}/monitors/{id}/checkins (paginated).
• MCP: new list_monitors tool.
• Retention purge covers check-in history; monitor state rows are kept. alert_history.issue_id stores monitor:<slug> for monitor alerts.

Why

Closes #360. Scheduled jobs (backups, workers, crons) fail silently — error tracking alone never sees a job that never ran. Any Sentry SDK can now report monitor check-ins via the same drop-in DSN swap; staleness alerts flow to the existing Discord/Slack webhooks.

Testing

• cargo fmt --all -- --check clean.
• cargo clippy --workspace --all-targets -- -D warnings clean.
• cargo test --workspace green: trapfalld unit 92, integration 43, MCP 17 — includes new tests: envelope check_in parse, staleness evaluation (fixed timestamps), end-to-end ingest→monitor list→checkins pagination (integration, real envelope POST with sentry-key auth), monitor-missed webhook dispatch + cooldown (offline), retention purge deletes old check-ins / keeps monitors, MCP list_monitors.
• Postgres dialect compiled under --features postgres (clippy clean); live-PG path exercised by the CI test-postgres job.
• Gates independently re-executed by the orchestrator, not taken from the implementer's self-report.

Limitations (documented in code): timezone field ignored (UTC), interval-type schedules not evaluated, monitor slug length capped at 128.