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/store/sqlite.go

@@ -168,6 +168,15 @@ func (d *SQLiteDialect) ImportWipe(tx *sql.Tx) {
 	if _, err := tx.Exec("DELETE FROM sqlite_sequence WHERE name='maintenance_windows'"); err != nil {
 		log.Printf("import wipe error: %v", err)
 	}
+	if _, err := tx.Exec("DELETE FROM check_history"); err != nil {
+		log.Printf("import wipe error: %v", err)
+	}
+	if _, err := tx.Exec("DELETE FROM state_changes"); err != nil {
+		log.Printf("import wipe error: %v", err)
+	}
+	if _, err := tx.Exec("DELETE FROM alert_health"); err != nil {
+		log.Printf("import wipe error: %v", err)
+	}
 }
 
 func (d *SQLiteDialect) ImportResetSequences(tx *sql.Tx) {}