feat: add reverse proxy support with X-Forwarded-Host header handling for agent installer URL generation

Add external_base_url helper to detect base URL from X-Forwarded-Host and X-Forwarded-Proto headers with fallback to request.base_url, replace hardcoded request.base_url usage in node_agent_install, node_agent_install_info, and public_node_agent_install endpoints to support reverse proxy deployments, update nginx config to pass X-Forwarded-Host header and use $http_host instead of $host for proper hostname forw

nginx/default.conf

@@ -4,9 +4,10 @@ server {
 
   location /api/ {
     proxy_pass http://api:8000/api/;
-    proxy_set_header Host $host;
+    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;
   }
 
@@ -16,7 +17,6 @@ server {
 
   location / {
     proxy_pass http://frontend:80;
-    proxy_set_header Host $host;
+    proxy_set_header Host $http_host;
   }
 }
-