Security hardening: CSRF, rate limiting, session/password policy, audit log

Fixes critical pre-existing issues found during review: bulk_action.php had no
auth check at all (unauthenticated download/delete of any qrcode) and built a
table name from unwhitelisted user input (SQL injection); the QR generator
classes wrote files from unvalidated filename/format, allowing path traversal
and arbitrary file writes. Also pins chillerlan/php-qrcode to 5.0.5 since
master now requires PHP 8.4, breaking the PHP 8.3 build.

- CSRF tokens on all POST forms and the bulk_action.php JSON endpoint
- Login rate limiting (5 attempts / 15 min) via new login_attempts table
- Hardened sessions: httponly/samesite cookies, 30 min idle timeout,
  session regeneration on login
- Forced password change for the default superadmin/superadmin account
- Server-side validation in Users/DynamicQrcode/Qrcode classes
- Audit log table for auth, user, and qrcode actions
- Checked-in db schema (db/init.sql, migrations/) instead of relying on an
  opaque prebuilt db image
- Production docker-compose with Nginx + php-fpm instead of the PHP dev server

src/dynamic_qrcode.php

@@ -1,11 +1,14 @@
 <?php
-session_start();
-require_once 'config/config.php';
+require_once 'includes/bootstrap.php';
 require_once BASE_PATH.'/includes/auth_validate.php';
 require_once BASE_PATH . '/lib/DynamicQrcode/DynamicQrcode.php';
 
 $dynamic_qrcode_instance = new DynamicQrcode();
 
+if ($_SERVER['REQUEST_METHOD'] === 'POST') {
+    csrf_verify_or_die();
+}
+
 $edit = false;
 if($_SERVER["REQUEST_METHOD"] === "GET" && isset($_GET["edit"]) && $_GET["edit"] == "true" && isset($_GET["id"])) {
     $edit = true;
@@ -83,6 +86,7 @@ if($_SERVER["REQUEST_METHOD"] === "POST" && !isset($_POST["edit"])) {
                     <h3 class="card-title">Enter the requested data</h3>
                 </div>
                 <form class="form" action="" method="post" id="dynamic_form" enctype="multipart/form-data">
+<?php echo csrf_field(); ?>
                     <div class="card-body">
                         <?php
                             if($edit)