Security: cap MCP HTTP request bodies and add optional Origin allowlist (#280)

Reject request bodies above 16 MiB with 413 before allocating, stopping an
unauthenticated Content-Length OOM (vec![0u8; len]). Add an optional,
default-off OBSCURA_MCP_ALLOWED_ORIGINS allowlist; unset keeps the permissive
default so issue #175 hosted dashboards are unaffected.

crates/obscura-mcp/tests/cors_preflight.rs

@@ -84,3 +84,54 @@ async fn options_preflight_lists_required_browser_headers() {
     })
     .await;
 }
+
+/// Regression test for the unbounded `Content-Length` allocation: a POST that
+/// advertises a huge body must be rejected with 413 *before* the server tries
+/// to allocate `vec![0u8; len]`, rather than committing gigabytes of RAM and
+/// OOM-ing the process (unauthenticated DoS).
+#[tokio::test(flavor = "current_thread")]
+async fn oversized_content_length_is_rejected() {
+    let port = pick_free_port();
+    let local = LocalSet::new();
+
+    let server = local.spawn_local(async move {
+        let _ = obscura_mcp::http::run("127.0.0.1".to_string(), port, None, None, false).await;
+    });
+
+    local
+        .run_until(async {
+            for _ in 0..40 {
+                if TcpStream::connect(("127.0.0.1", port)).await.is_ok() {
+                    break;
+                }
+                sleep(Duration::from_millis(50)).await;
+            }
+
+            let mut stream = TcpStream::connect(("127.0.0.1", port))
+                .await
+                .expect("MCP server did not come up");
+            // 8 GiB advertised, zero body sent. Pre-fix the server allocates 8 GiB.
+            let req = b"POST /mcp HTTP/1.1\r\n\
+                        Host: 127.0.0.1\r\n\
+                        Content-Type: application/json\r\n\
+                        Content-Length: 8589934592\r\n\
+                        \r\n";
+            stream.write_all(req).await.unwrap();
+            stream.flush().await.unwrap();
+
+            let mut buf = [0u8; 1024];
+            let n = timeout(Duration::from_secs(2), stream.read(&mut buf))
+                .await
+                .expect("read timed out")
+                .expect("read failed");
+            let response = String::from_utf8_lossy(&buf[..n]).to_string();
+
+            server.abort();
+
+            assert!(
+                response.starts_with("HTTP/1.1 413"),
+                "expected 413 Payload Too Large, got:\n{response}"
+            );
+        })
+        .await;
+}