feat(auth): add MINDA_DEMO support with auto-seeded demo accounts and username login

docker-compose.yml

@@ -14,6 +14,8 @@ services:
       - BASE_URL=${BASE_URL:-http://localhost:8080}
       - ALLOW_REGISTRATION=${ALLOW_REGISTRATION:-false}
       - LOG_LEVEL=${LOG_LEVEL:-info}
+      # Demo mode: if true, automatically seeds demo accounts admin/admin and user/user
+      - MINDA_DEMO=${MINDA_DEMO:-false}
       # Optional SMTP / Mail Configuration (can also be configured in Admin UI)
       - SMTP_HOST=${SMTP_HOST:-}
       - SMTP_PORT=${SMTP_PORT:-587}