Function create_router 

pub fn create_router(
    processor: Arc<SignalProcessor>,
    api_keys: Vec<ApiKeyConfig>,
    cors_enabled: bool,
) -> Router

Build the axum router with all routes.

api_keys is taken from BrainConfig.access.api_keys by the caller. When cors_enabled is true, CORS is restricted to localhost origins (Brain is a local daemon). When false, no CORS layer is applied (useful for reverse-proxy setups that handle CORS externally).