feat(backend): Add configuration validation

[activatedkc]

Problem

Missing environment variables cause runtime errors in the backend, often only discovered when a specific service (like AI verification) is invoked.

Proposed Solution

Implemented centralized environment variable validation at application startup using Zod.

• Centralized Config: Added backend/src/config/env.ts to define the environment schema and defaults.
• Startup Validation: The backend now validates all required variables (specifically OPENAI_API_KEY) immediately upon starting. If validation fails, the process exits with a clear error message.
• Typed Configuration: Refactored stellar, verification, and invoice services to use a unified, typed config() object instead of raw process.env calls.
• Documentation: Updated ENV_VARS.md and .env.example to clearly state which variables are required.

Acceptance Criteria

• Check required env vars on startup
• Return clear error messages for missing vars
• Document required variables

Technical Notes

• New File: backend/src/config/env.ts
• Modified Files:
  • backend/src/index.ts
  • backend/src/services/stellar.ts
  • backend/src/services/verification.ts
  • backend/src/services/invoice.ts
  • ENV_VARS.md
  • backend/.env.example

Fixes #20

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[activatedkc]

Kindly review @Smartdevs17

[activatedkc]

The PR is ready for review @Smartdevs17

[Smartdevs17]

merge conflict bro