fix(server-php): add minimal product placeholder for base image

The Dockerfile requires product/ directory for the builder stage.
For the base image, this is a minimal placeholder that returns JSON.
Applications should mount their own code at /var/www/html.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server-php/product/composer.json

@@ -0,0 +1,6 @@
+{
+    "name": "host-uk/server-php-placeholder",
+    "description": "Placeholder for base image",
+    "type": "project",
+    "require": {}
+}

server-php/product/public/index.php

@@ -0,0 +1,8 @@
+<?php
+// Placeholder for base image
+// Applications should mount/copy their own code
+echo json_encode([
+    'status' => 'ok',
+    'message' => 'Host UK Server PHP Base Image',
+    'note' => 'Mount your Laravel app to /var/www/html'
+]);