feat(mcp): add --host flag for MCP HTTP transport bind address (#254)

Add a --host CLI flag to 'obscura mcp --http' so the server can bind to
addresses other than 127.0.0.1 (e.g. 0.0.0.0 for Docker Compose sidecar
deployments). The default remains 127.0.0.1 for security.

Closes #253

crates/obscura-mcp/tests/cors_preflight.rs

@@ -27,7 +27,7 @@ async fn options_preflight_lists_required_browser_headers() {
     // end of the test. `current_thread` + LocalSet is required because the
     // browser state is `!Send` (Page holds V8 handles).
     let server = local.spawn_local(async move {
-        let _ = obscura_mcp::http::run(port, None, None, false).await;
+        let _ = obscura_mcp::http::run("127.0.0.1".to_string(), port, None, None, false).await;
     });
 
     local.run_until(async {