feat(sep-1): serve stellar.toml for ORGUSD

[pope-h]

Summary

Implements SEP-0001 asset metadata hosting for the ORGUSD asset by serving a stellar.toml at /.well-known/stellar.toml from the backend, with CORS enabled for wallet/explorer discovery.

Changes

• Add GET /.well-known/stellar.toml endpoint in backend/src/index.ts
  • Returns TOML content including:
    • VERSION
    • NETWORK_PASSPHRASE (when STELLAR_NETWORK_PASSPHRASE is set)
    • [DOCUMENTATION] org metadata + contact details
    • [[CURRENCIES]] entry for ORGUSD using ORGUSD_ISSUER_PUBLIC
  • Sets required headers:
    • Access-Control-Allow-Origin: *
    • Content-Type: text/plain; charset=utf-8

Configuration

This endpoint requires:

• ORGUSD_ISSUER_PUBLIC (required)
• STELLAR_NETWORK_PASSPHRASE (optional but recommended)

If ORGUSD_ISSUER_PUBLIC is missing, the endpoint returns 503 Service Unavailable with a clear error message.

Validation

1. Run the backend with ORGUSD_ISSUER_PUBLIC set (from backend/scripts/issue-orgusd.ts output).
2. Visit:
   • http://localhost:<PORT>/.well-known/stellar.toml
3. Validate the result with:
   • Stellar Expert (domain/TOML discovery)
   • Any TOML/SEP-1 checker

Fixes #34