Admin-scoped user accounts, secure qr code storage, PHP 8.4 upgrade

Admin-scoped users (answers: who can create a 'user' account, only super or
also an admin within their own scope?):
- New owner_admin_id column on users (migration 004). NULL means created by
  super (company-wide, previous behavior); otherwise scoped to that admin's
  own codes.
- Users::addUser/editUser/deleteUser now allow an 'admin' session, but force
  type='user' and owner_admin_id to their own id regardless of submitted
  input. user.php/users.php open up to admins with a restricted UI (no type
  picker, listing limited to their own created users).
- New qr_compute_scope_owner_id()/qr_apply_owner_scope()/qr_has_full_visibility()
  helpers in includes/security.php, replacing the ad-hoc type==='admin' checks
  in index.php, dynamic_qrcodes.php, static_qrcodes.php and bulk_action.php.
  A 'user' account created by an admin is now scoped to that admin's codes
  instead of seeing everything company-wide.

Qr code storage hardening: images were served as plain static files under the
document root with no auth check at all. Storage now lives outside the web
root; qrcode_image.php and qrcode_zip_download.php gate access with the same
permission model as the list pages, and the bulk zip download is bound to the
session that generated it.

PHP 8.4 + chillerlan/php-qrcode 6.0.1: bumped since this is a dockerized app,
so the PHP version shipped doesn't matter to end users. Note: the 6.0.1 tag
itself only requires PHP 8.2 - the earlier "needs 8.4" read was from an
unpinned clone of master, which has since moved past the tag. Fixed along the
way, surfaced by testing on 8.4:
- The hardcoded Imagick build (an old pinned master commit, workaround for
  3.7.0 being broken on PHP 8.3+) no longer compiles on 8.4. Imagick 3.8.1 is
  now a normal stable release, so the workaround is gone.
- config.php had display_errors=On + error_reporting(E_ALL), so PHP 8.4's new
  deprecation notices got dumped straight into the response before
  session_start() could run, breaking login outright. Also an info-disclosure
  risk on its own. Now logged instead of displayed.
- MysqliDb::insertMulti() had an implicit nullable parameter, now explicit.
- includes/auth_validate.php redirected unauthenticated requests but never
  called exit(), so the rest of the script kept running.
- Dockerfile.fpm was missing both git (needed to clone chillerlan/php-qrcode)
  and the imagick extension entirely.

Also removes the unused sample qr code images that shipped in the original
repo; storage now lives outside the document root so they were never going
to be served again.

Dockerfile.fpm

@@ -1,4 +1,4 @@
-FROM php:8.3-fpm
+FROM php:8.4-fpm
 
 RUN if [ "$(grep '^VERSION_ID=' /etc/os-release | cut -d '=' -f 2 | tr -d '"')" -eq "9" ]; then \
         sed -i -e 's/deb.debian.org/archive.debian.org/g' \
@@ -13,6 +13,7 @@ RUN chmod +x /usr/local/bin/install-php-extensions
 RUN DEBIAN_FRONTEND=noninteractive apt-get update -q \
     && DEBIAN_FRONTEND=noninteractive apt-get install -qq -y \
       curl \
+      git \
       libzip-dev \
       libjpeg62-turbo-dev \
       libpng-dev \
@@ -21,6 +22,7 @@ RUN DEBIAN_FRONTEND=noninteractive apt-get update -q \
     && install-php-extensions \
       gd \
       gettext \
+      imagick \
       intl \
       mysqli \
       opcache \
@@ -41,8 +43,8 @@ RUN cd /opt \
 
 RUN mkdir -p /opt && chmod 777 /opt
 WORKDIR /opt
-# Zie Dockerfile: vastgezet op 5.0.5, want 6.0.0+ vereist PHP >= 8.4.
-RUN git clone --branch 5.0.5 --depth 1 https://github.com/chillerlan/php-qrcode.git \
+# See Dockerfile: pinned to a specific release tag instead of an unpinned clone of master.
+RUN git clone --branch 6.0.1 --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 cp -R ./php-qrcode/src /var/www/html/
@@ -52,8 +54,13 @@ RUN composer update
 COPY ./src ./
 RUN chown -R www-data:www-data /var/www/html \
     && find /var/www/html -type f -exec chmod 644 {} \; \
-    && find /var/www/html -type d -exec chmod 755 {} \; \
-    && chmod -R 775 /var/www/html/saved_qrcode
+    && find /var/www/html -type d -exec chmod 755 {} \;
+
+# Qr code storage lives outside the document root so files can only be reached through
+# the authenticated qrcode_image.php / qrcode_zip_download.php endpoints.
+RUN mkdir -p /var/www/qrcode-storage/zip \
+    && chown -R www-data:www-data /var/www/qrcode-storage \
+    && chmod -R 775 /var/www/qrcode-storage
 
 EXPOSE 9000
 CMD ["php-fpm"]