#55 Docs: expand .env.example for multi-key auth

[Anonymous]

Originally created by: yaront1111

Context

.env.example only includes a single API key, but the gateway supports CORDUM_API_KEYS and CORDUM_API_KEYS_PATH for multi-key setups with roles/tenants.

Task

Expand .env.example with minimal examples for multi-key configuration.

Acceptance criteria

• Add a short example CORDUM_API_KEYS line (comma-separated and/or JSON).
• Add an example CORDUM_API_KEYS_PATH line.
• Keep comments short and consistent with existing .env.example style.