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>

.env.example

@@ -0,0 +1,15 @@
+# Kopieer naar .env en pas de waarden aan. .env wordt niet gecommit (zie .gitignore).
+
+TYPE=docker
+QRCODE_GENERATOR=internal-chillerlan.qrcode
+BASE_URL=http://localhost
+
+DATABASE_HOST=php-dynamic-qrcode-db
+DATABASE_PORT=3306
+DATABASE_NAME=qrcode
+DATABASE_USER=qrcode
+DATABASE_PASSWORD=change-me-to-a-strong-password
+DATABASE_PREFIX=
+DATABASE_CHARSET=utf8
+
+MYSQL_ROOT_PASSWORD=change-me-to-a-strong-root-password