[BE-56] Add environment-specific configuration with startup validation

[mftee]

Overview

The app has a single .env file with no distinction between development, staging, and production environments. Environment-specific configs must differ per environment.

Acceptance Criteria

• Support .env.development, .env.staging, .env.production loaded based on NODE_ENV
• Add a ConfigValidationSchema using Joi that validates all required variables at startup
• STELLAR_NETWORK defaults to testnet in development and must be explicitly set to public in production
• LOG_LEVEL defaults to debug in development and warn in production
• App refuses to start with a clear error if any required variable is missing or invalid