fix: Update nginx upstream name from php-fpm to php-upstream

Ensure consistency with nginx base image configuration

deployment/stacks/application/nginx/conf.d/default.conf

@@ -2,7 +2,7 @@
 # Optimized for production with security headers and performance tuning
 
 # Upstream PHP-FPM
-upstream php-fpm {
+upstream php-upstream {
     server app:9000;
 }
 
@@ -90,7 +90,7 @@ server {
         try_files $uri =404;
         fastcgi_split_path_info ^(.+\.php)(/.+)$;
 
-        fastcgi_pass php-fpm;
+        fastcgi_pass php-upstream;
         fastcgi_index index.php;
         fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
         fastcgi_param PATH_INFO $fastcgi_path_info;