chore(backend): add environment variable validation with Joi

[nonoto999t]

Summary

Add environment variable validation using Joi for NestJS backend.

Changes

Validation Schema

• Created env.validation.ts with Joi schema
• Validates: DATABASE_URL, JWT_SECRET, STELLAR_NETWORK, PORT, FRONTEND_URL
• Custom error messages for each required field
• Contract ID validation with regex pattern

Configuration

• Updated .env.example with all required fields and defaults
• Uses ConfigModule.forRoot with validationSchema
• Fail fast with clear error messages if env vars are missing

Closes #12