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/config/mod.rs

@@ -108,6 +108,7 @@ pub struct SystemdConfig {
     pub nginx_check_interval_seconds: u64,
     pub http_timeout_seconds: u64,
     pub http_connect_timeout_seconds: u64,
+    pub nginx_latency_critical_ms: f32,
 }