feat(infra): configure default load test environment

[tasoo-oos]

Description

K6 기반 로드테스트에서 사용될 기본 세팅을 .env.example에 넣었습니다.

Additional context

Closes TAS-2724

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[gemini-code-assist]

Code Review

This pull request introduces support for configuring load tests via a .env file, reducing the default load and allowing users to customize the number of virtual users (VUs) and ramp-up/steady/ramp-down durations. However, there is a critical issue in submission-load.js where open('./.env') is evaluated statically by k6 during initialization. If the .env file does not exist, k6 will throw a fatal error and crash before the try...catch block can execute, which breaks execution when only using environment variables.