Rename internal Docker service/network/volume identifiers to qrforge-*

Cosmetic-only rename, no functional/data-layer change: service names
(php-dynamic-qrcode -> qrforge-app, php-dynamic-qrcode-db -> qrforge-db),
the network (php-dynamic-qrcode-network -> qrforge-network), and the two
named volumes (php_dynamic_qrcode_db_data -> qrforge_db_data,
php_dynamic_qrcode_saved_qrcode_data -> qrforge_qrcode_storage), plus
matching references in nginx.conf's fastcgi_pass and .env.example's
DATABASE_HOST default. Database name itself ('qrcode') intentionally
left unchanged per user request.

Volume rename requires an explicit data migration on already-deployed
hosts (a bare name change would otherwise attach a fresh empty volume) -
handled separately as part of this same deploy, not by this commit.

nginx.conf

@@ -15,7 +15,7 @@ server {
     }
 
     location ~ \.php$ {
-        fastcgi_pass php-dynamic-qrcode:9000;
+        fastcgi_pass qrforge-app:9000;
         fastcgi_index index.php;
         include fastcgi_params;
         fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;