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/forms/form_users.php

@@ -20,7 +20,7 @@
                     <span class="input-group-text"><i class="fa fa-lock"></i></span>
                 </div>
                 
-                <input type="password" name="password" placeholder="Password" class="form-control" required="required" autocomplete="off">
+                <input type="password" name="password" placeholder="<?php echo ($edit) ? 'Leave blank to keep current password' : 'Password'; ?>" class="form-control" <?php echo ($edit) ? '' : 'required="required"'; ?> minlength="10" autocomplete="off">
             </div>
         </div>
     </div>