feat(nginx): generate 502 config for apps without web listeners

When an app has no web listeners (not yet deployed, no web process type,
or web processes not running), generate a minimal nginx config that
returns 502 Bad Gateway instead of having no config at all. This ensures
domains resolve, monitoring tools detect non-200 status codes, and SSL
certificate provisioning tools like letsencrypt can function.

plugins/nginx-vhosts/post-create

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+source "$PLUGIN_CORE_AVAILABLE_PATH/common/property-functions"
+source "$PLUGIN_AVAILABLE_PATH/nginx-vhosts/functions"
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+trigger-nginx-vhosts-post-create() {
+  declare desc="nginx-vhosts post-create trigger"
+  declare trigger="post-create"
+  declare APP="$1"
+
+  fn-plugin-property-setup-app "nginx" "$APP"
+
+  if [[ "$(plugn trigger proxy-type "$APP")" != "nginx" ]]; then
+    return
+  fi
+
+  nginx_build_config "$APP"
+}
+
+trigger-nginx-vhosts-post-create "$@"