fix: revert @types/node to v20 to match production Node version

Production server (vmi2736502) uses Node v20.20.0, not v22.
@types/node must match the lowest Node version we support.

Changed:
- @types/node: ^22.12.5 → ^20.19.33

Local dev may use Node 22, but types must be compatible with
production Node 20 to avoid runtime errors from missing APIs.

https://claude.ai/code/session_015R3U3kiprQiNTTNNt31Sg6

package.json

@@ -59,7 +59,7 @@
     "@eslint/eslintrc": "^3.3.1",
     "@eslint/js": "^9.31.0",
     "@types/jest": "^30.0.0",
-    "@types/node": "^22.12.5",
+    "@types/node": "^20.19.33",
     "@types/ws": "^8.0.0",
     "@typescript-eslint/eslint-plugin": "^8.55.0",
     "@typescript-eslint/parser": "^8.55.0",