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

Cargo.lock

@@ -270,7 +270,7 @@ checksum = "a1d728cc89cf3aee9ff92b05e62b19ee65a02b5702cff7d5a377e32c6ae29d8d"
 
 [[package]]
 name = "cm-dashboard"
-version = "0.1.35"
+version = "0.1.37"
 dependencies = [
  "anyhow",
  "chrono",
@@ -291,7 +291,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-agent"
-version = "0.1.35"
+version = "0.1.37"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -314,7 +314,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-shared"
-version = "0.1.35"
+version = "0.1.37"
 dependencies = [
  "chrono",
  "serde",