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

Server/storage/main_test.go

@@ -0,0 +1,11 @@
+package storage_test
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+)
+
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m)
+}