refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/pkg/util/password_test.go

@@ -0,0 +1,23 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import "testing"
+
+func TestDummyCheckPasswordDoesNotPanic(t *testing.T) {
+	DummyCheckPassword("any-password")
+}
+
+func TestCheckPasswordHashRoundTrip(t *testing.T) {
+	hash, err := HashPassword("secret-pass")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !CheckPasswordHash(hash, "secret-pass") {
+		t.Fatal("expected matching password to succeed")
+	}
+	if CheckPasswordHash(hash, "other-pass") {
+		t.Fatal("expected mismatched password to fail")
+	}
+}