Fix ProxyFCGISetEnvIf syntax for SCRIPT_FILENAME rewrite

The previous expr= with s|...|...| substitution syntax doesn't exist
in Apache expressions — it silently failed, leaving SCRIPT_FILENAME
pointing to /mnt/users/ which PHP-FPM can't find.

Fixed to use regex match in the conditional with backreferences:
  reqenv('SCRIPT_FILENAME') =~ m#^/mnt/users/([^/]+)/([^/]+)/public_html(.*)#
  -> /home/$1/public_html$3

This is also generic (captures user from the path) so the template
no longer needs per-user placeholder substitution for this directive.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

configs/shared-vhost-template.tpl

@@ -33,9 +33,9 @@
 
     # Rewrite SCRIPT_FILENAME for PHP-FPM: the shared httpd serves from
     # /mnt/users/{user}/{domain}/public_html but the FPM container has
-    # files at /home/{user}/public_html. Replace the base path so FPM
+    # files at /home/{user}/public_html. Use regex match with backreferences
-    # can find the PHP files at its local mount point.
+    # to reconstruct the correct path for the FPM container.
-    ProxyFCGISetEnvIf "true" SCRIPT_FILENAME "expr=%{reqenv:SCRIPT_FILENAME} =~ s|/mnt/users/~~user~~/~~domain~~/public_html|/home/~~user~~/public_html|"
+    ProxyFCGISetEnvIf "reqenv('SCRIPT_FILENAME') =~ m#^/mnt/users/([^/]+)/([^/]+)/public_html(.*)#" SCRIPT_FILENAME "/home/$1/public_html$3"
 
     DirectoryIndex index.php index.html index.htm