feat(cluster): add probe execution mode, check extraction, and result aggregation

Phase 2 of distributed probing:
- Extract check logic into standalone RunCheck() for use by probes
- Add probe cluster mode: stateless nodes that fetch assignments, execute
  checks, and report results to the leader
- Add multi-node result aggregation with configurable strategy
  (any-down, majority-down, all-down)
- Leader ingests probe results into engine live state and triggers alerts
- New env vars: UPKEEP_NODE_ID, UPKEEP_NODE_NAME, UPKEEP_NODE_REGION,
  UPKEEP_AGG_STRATEGY
- Example docker-compose.probe.yml with leader + 2 regional probes

internal/server/server.go

@@ -325,6 +325,7 @@ func Start(cfg ServerConfig, s store.Store, eng *monitor.Engine) {
 			if err := s.SaveCheckFromNode(result.SiteID, req.NodeID, result.LatencyNs, result.IsUp); err != nil {
 				log.Printf("Failed to save probe result: %v", err)
 			}
+			eng.IngestProbeResult(req.NodeID, result.SiteID, result.LatencyNs, result.IsUp)
 		}
 		s.UpdateNodeLastSeen(req.NodeID)
 		json.NewEncoder(w).Encode(map[string]bool{"ok": true})