Make nginx site latency thresholds configurable and simplify status logic

- Replace hardcoded 500ms/2000ms thresholds with configurable nginx_latency_critical_ms
- Simplify status logic to only OK or Critical (no Warning status)
- Add validation for nginx latency threshold configuration
- Re-enable nginx site collection with configurable thresholds
- Resolves issue where sites showed critical at 2000ms despite 30s timeout setting
- Bump version to v0.1.38

agent/src/collectors/systemd.rs

@@ -555,10 +555,8 @@ impl SystemdCollector {
         for (site_name, url) in &sites {
             match self.check_site_latency(url) {
                 Ok(latency_ms) => {
-                    let status = if latency_ms < 500.0 {
+                    let status = if latency_ms < self.config.nginx_latency_critical_ms {
                         Status::Ok
-                    } else if latency_ms < 2000.0 {
-                        Status::Warning
                     } else {
                         Status::Critical
                     };