feat: add /agents/ nginx proxy route and improve agent heartbeat error handling with URL normalization

Add /agents/ location block to nginx config to proxy agent heartbeat requests directly to backend API without /api/v1 prefix duplication, implement normalized_api_url helper to detect and fix double /api/v1 suffixes in agent config with automatic /api/v1 appending when missing, enhance heartbeat error logging to include HTTP status codes, response body preview, and resolved API URL for debugging connection

nginx/default.conf

@@ -11,6 +11,15 @@ server {
     proxy_set_header X-Forwarded-Proto $scheme;
   }
 
+  location /agents/ {
+    proxy_pass http://api:8000/api/v1/agents/;
+    proxy_set_header Host $http_host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    proxy_set_header X-Forwarded-Host $http_host;
+    proxy_set_header X-Forwarded-Proto $scheme;
+  }
+
   location /healthz {
     proxy_pass http://api:8000/healthz;
   }