#45 Issue 45: [Architecture] Support Custom RPC Providers


Description:
Do not hardcode public Horizon endpoints. Allow DAOs running self-hosted backends to plug in their own RPC URLs (like QuickNode or private nodes).
Acceptance Criteria:

[ ] Ensure all Stellar SDK initializations pull from process.env.STELLAR_RPC_URL.

[ ] Document this requirement in .env.example.
Labels: architecture, ops