fix(engine): six correctness fixes for the state machine

1. Group auto-pause trap: remove the one-way Paused=true mutation
   from checkGroup — monitorRoutine skipped paused groups, so they
   could never re-evaluate or auto-unpause.

2. Retry logic: apply MaxRetries to all →DOWN transitions, not just
   UP→DOWN. New monitors (PENDING) no longer alert on first transient
   failure when retries are configured.

3. Shutdown drain hole: track checker goroutines with checkerWG so
   Stop() waits for in-flight checks before draining the write queue.
   Final drainWrites() catches any writes enqueued after the writer's
   own drain.

4. Probe-ingest writer bypass: route SaveCheckFromNode through the
   engine's serialized dbWriter instead of writing directly to the
   store from the HTTP handler.

5. Dead-probe expiry: expire stale probe results (>3× site interval)
   before aggregation so a dead probe can't poison status forever.
   Also clean probeResults in RemoveSite.

6. Maintenance-cache N+1: replace per-check DB query with a
   fully-resolved in-memory cache refreshed every poll cycle. One
   GetActiveMaintenanceWindows() call instead of N IsMonitorInMaintenance.

ImportData now wipes check_history, state_changes, and alert_health
so re-inserted IDs don't inherit stale history from prior occupants.

internal/monitor/dbwriter.go

@@ -44,3 +44,15 @@ type writeAlertHealth struct{ rec models.AlertHealthRecord }
 
 func (w writeAlertHealth) exec(s store.Store) error { return s.SaveAlertHealth(w.rec) }
 func (w writeAlertHealth) desc() string             { return "alert-health" }
+
+type writeProbeCheck struct {
+	siteID    int
+	nodeID    string
+	latencyNs int64
+	isUp      bool
+}
+
+func (w writeProbeCheck) exec(s store.Store) error {
+	return s.SaveCheckFromNode(w.siteID, w.nodeID, w.latencyNs, w.isUp)
+}
+func (w writeProbeCheck) desc() string { return "probe-check" }