fix: remove sites-enabled default instead of renaming

Renaming `/etc/nginx/sites-enabled/default` to `.dokku-disabled` left a broken symlink that nginx's `include /etc/nginx/sites-enabled/*;` glob still picked up, breaking config validation. Files inside `sites-enabled/` are now removed; the real config in `sites-available/default` is still preserved via rename.

debian/postinst

@@ -135,6 +135,36 @@ setup-sshcommand() {
   fi
 }
 
+disable-default-vhost-path() {
+  local path="$1"
+  local backup_path="${path}.dokku-disabled"
+
+  if [ ! -e "$path" ] && [ ! -L "$path" ]; then
+    return
+  fi
+
+  if [ -e "$backup_path" ] || [ -L "$backup_path" ]; then
+    return
+  fi
+
+  case "$path" in
+    /etc/nginx/sites-enabled/*)
+      rm -f "$path"
+      echo "Removed conflicting nginx default at $path" 1>&2
+      return
+      ;;
+  esac
+
+  if [ -L "$path" ]; then
+    rm -f "$path"
+    echo "Removed conflicting nginx default symlink at $path" 1>&2
+    return
+  fi
+
+  mv -n "$path" "$backup_path"
+  echo "Disabled $path (renamed to $backup_path)" 1>&2
+}
+
 setup-default-site() {
   db_get "dokku/install_default_site"
   if [ "$RET" != "true" ]; then
@@ -153,10 +183,7 @@ setup-default-site() {
   fi
 
   for path in /etc/nginx/sites-enabled/default /etc/nginx/sites-available/default /etc/nginx/conf.d/default.conf; do
-    if [ -e "$path" ] && [ ! -e "${path}.dokku-disabled" ]; then
-      mv -n "$path" "${path}.dokku-disabled"
-      echo "Disabled $path (renamed to ${path}.dokku-disabled)" 1>&2
-    fi
+    disable-default-vhost-path "$path"
   done
 
   echo "Installing nginx default catch-all site at $default_vhost_target"