feat(env): add environment variable validation using Zod

[dev-kraken]

• Implemented a Zod schema to validate required environment variables: POSTGRES_URL, STRIPE_SECRET_KEY, STRIPE_WEBHOOK_SECRET, BASE_URL, and AUTH_SECRET.

• Added error handling to log validation issues and prevent application startup with invalid configurations.

• Ensured the validation logic is reusable and can be easily integrated into the project's configuration.

This feature improves application reliability by catching configuration errors early in the startup process.