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.

src/user.php

@@ -5,8 +5,8 @@ require_once BASE_PATH . '/lib/Users/Users.php';
 
 $user_instance = new Users();
 
-if ($_SESSION['type'] !== 'super')
-    $user_instance->failure('Only a "super admin" account can access the admin listing page', 'Location: index.php');
+if (!in_array($_SESSION['type'], ['super', 'admin'], true))
+    $user_instance->failure('Only "super admin" and "admin" accounts can access the user management page', 'Location: index.php');
 
 if ($_SERVER['REQUEST_METHOD'] === 'POST') {
     csrf_verify_or_die();
@@ -16,6 +16,13 @@ $edit = false;
 if($_SERVER["REQUEST_METHOD"] === "GET" && isset($_GET["edit"]) && $_GET["edit"] == "true" && isset($_GET["id"])) {
     $edit = true;
     $user = $user_instance->getUser($_GET["id"]);
+
+    // An admin may only open the edit form for their own 'user' accounts.
+    if ($_SESSION['type'] === 'admin' && (
+        $user['type'] !== 'user' || (int) $user['owner_admin_id'] !== (int) $_SESSION['user_id']
+    )) {
+        $user_instance->failure('You are not allowed to edit this user', 'Location: users.php');
+    }
 }
 
 if($_SERVER["REQUEST_METHOD"] === "POST" && isset($_POST["del_id"])) {