Fix stale qr-storage volume mount path in dev docker-compose.yml

Fase 1 hardening (2026-07-08) moved saved QR code storage from
src/saved_qrcode/ to /var/www/qrcode-storage/ (outside the webroot,
SAVED_QRCODE_DIRECTORY in config.php), but docker-compose.yml's volume
mount was never updated and still pointed at the old path
(/var/www/html/saved_qrcode). The named volume was therefore mounted
somewhere the app never wrote to - every actual qr code image the app
generates at /var/www/qrcode-storage/ lived only in the container's
ephemeral filesystem and was silently lost on every container
recreation, while the qrcode_storage volume itself stayed permanently
empty. DB rows (filenames/content) were never affected, only the
generated image files.

Confirmed as the cause of qr.ensembia.com's "old and new QR codes not
showing in the list" report: qrcode_image.php's is_file() check failed
because the file genuinely wasn't there anymore. docker-compose.prod.yml
already had the correct path - only the dev compose file (what
qr.ensembia.com actually runs) had this bug.

docker-compose.yml

@@ -21,7 +21,7 @@ services:
       php-dynamic-qrcode-db:
         condition: service_healthy
     volumes:
-      - php_dynamic_qrcode_saved_qrcode_data:/var/www/html/saved_qrcode
+      - php_dynamic_qrcode_saved_qrcode_data:/var/www/qrcode-storage
     networks:
       - php-dynamic-qrcode-network