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

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/forms/table_dynamic.php

@@ -3,6 +3,7 @@
         <div id="err-msg"></div>
         <div class="bulk-action-wrapper">
             <form id="bulk-action" action="bulk_action.php" method="POST">
+<?php echo csrf_field(); ?>
                 <div class="col-sm-12 mb-2" style="margin-left: 10px">
                     <div class="row">
                         <div class="col-5 col-md-2">
@@ -100,6 +101,7 @@
 <div class="modal fade" id="delete-modal" role="dialog">
     <div class="modal-dialog">
         <form action="dynamic_qrcode.php" method="POST">
+<?php echo csrf_field(); ?>
             <!-- Modal content -->
 
             <div class="modal-content">