Fix nginx sub-services flickering with persistent caching

- Remove nginx_ prefix from site names in hierarchical structure
- Fix get_nginx_site_metrics to call correct internal method
- Implement same caching functionality as old working version
- Sites now stay visible continuously with 30s latency updates
- Preserve cached results between refresh cycles

Cargo.lock

@@ -279,7 +279,7 @@ checksum = "a1d728cc89cf3aee9ff92b05e62b19ee65a02b5702cff7d5a377e32c6ae29d8d"
 
 [[package]]
 name = "cm-dashboard"
-version = "0.1.145"
+version = "0.1.146"
 dependencies = [
  "anyhow",
  "chrono",
@@ -301,7 +301,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-agent"
-version = "0.1.145"
+version = "0.1.146"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -324,7 +324,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-shared"
-version = "0.1.145"
+version = "0.1.146"
 dependencies = [
  "chrono",
  "serde",