feat: production-grade routing cache + stale detection fixes

Three-tier routing cache (ESL global -> file -> HTTP) eliminates HTTP
from call path for 2500-5000 concurrent calls. Lua reads cached route
in ~0.1us instead of blocking on api:execute('curl', ...).

Engine fixes:
- recalcBestLocked now skips stale nodes (was 5 min bug -> now ~10-15s)
- PickNodeForCall action_type='node' checks staleness for consistency
- onMetric callback pushes cache on every metric (no ticker delay)

Config:
- stale_threshold_sec default 20 -> 10 (industry standard)
- contrib/ included in Makefile deploy target
- route.lua paths updated for /opt/pulse-lets-go

cmd/pulse-lets-go/main.go

@@ -139,8 +139,13 @@ func main() {
 
 	// 6. HTTP API
 	apiHandler = api.NewAPI(eng, cfgMgr, cfg.GetJWTSecret(), cfg.MonitoringAPIKey, sub.IsConnected, cfg)
+	apiHandler.SetRouteCachePath(filepath.Join(dataDir, "route_cache.json"))
 	apiHandler.RebuildTrunkCache() // первичное построение in-memory кэша gateway
 
+	sub.SetOnMetric(func() {
+		apiHandler.PushRouteCache()
+	})
+
 	// При эвикции stale-ноды — удаляем её balance-транк из trunks.json
 	eng.SetOnNodeEvicted(func(nodeID string) {
 		trunks, err := cfgMgr.ReadTrunks()
@@ -232,6 +237,7 @@ func main() {
 	go func() {
 		ticker := time.NewTicker(5 * time.Second)
 		for range ticker.C {
+			apiHandler.PushRouteCache()
 			if eslClient != nil && eslClient.IsConnected() {
 				if stats, err := eslClient.FetchFsStats(); err == nil {
 					apiHandler.UpdateFsStats(stats)