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

internal/config/manager.go

@@ -420,7 +420,7 @@ func (m *Manager) createDefaultConfig() error {
 		ListenAddr:        ":8080",
 		JWTSecret:         jwtSecret,
 		MonitoringAPIKey:  apiKey,
-		StaleThresholdSec: 20,
+		StaleThresholdSec: 10,
 		Scoring: ScoringConfig{
 			IdleCPUMin:            5.0,
 			CallFailureRateLethal: 15.0,