feat: add syncutil mutex, test scaffolding, and server hardening

- Add syncutil package with deadlock-detecting mutex (build-tag switchable)
- Add main_test.go TestMain scaffolding across all server packages
- Harden concurrency in ws, admin, auth, and updater packages
- Update CI workflow, go.mod/sum, Cargo.lock, and root changelogen tooling

package.json

@@ -0,0 +1,10 @@
+{
+  "private": true,
+  "scripts": {
+    "changelog": "changelogen",
+    "release": "changelogen --release"
+  },
+  "devDependencies": {
+    "changelogen": "^0.6.2"
+  }
+}