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/dist/js/custom.js

@@ -470,6 +470,9 @@
       data: JSON.stringify(data),
       dataType: "json",
       contentType: 'application/json',
+      headers: {
+        'X-CSRF-Token': $('meta[name="csrf-token"]').attr('content')
+      },
       success: (res) => {
         if (res.status == 200) {
           if(data["action"] === "download") {