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

Dockerfile

@@ -86,7 +86,10 @@ RUN docker-php-ext-install sockets && docker-php-ext-enable sockets
 
 RUN mkdir -p /opt && chmod 777 /opt
 WORKDIR /opt
-RUN git clone https://github.com/chillerlan/php-qrcode.git \
+# Vastgezet op 5.0.5 (laatste 5.x-release): vanaf 6.0.0 vereist de library PHP >= 8.4,
+# terwijl deze image op PHP 8.3 draait. Een ongepinde clone van master is bovendien
+# een reproduceerbaarheids-/supply-chain-risico (build kan zonder waarschuwing breken).
+RUN git clone --branch 5.0.5 --depth 1 https://github.com/chillerlan/php-qrcode.git \
         && chmod -R 777 ./php-qrcode
 RUN cp ./php-qrcode/composer.json /var/www/html/composer.json
 RUN mkdir -p /var/www/html/test && chmod 777 /var/www/html/test