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/plugins/infra/cache/redis_test.go

@@ -0,0 +1,28 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cache
+
+import (
+	"testing"
+
+	"github.com/redis/go-redis/v9/maintnotifications"
+)
+
+func TestRedisMaintNotificationsConfig(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		enabled bool
+		want    maintnotifications.Mode
+	}{
+		{name: "disabled by default", enabled: false, want: maintnotifications.ModeDisabled},
+		{name: "auto when enabled", enabled: true, want: maintnotifications.ModeAuto},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			cfg := redisMaintNotificationsConfig(test.enabled)
+			if cfg.Mode != test.want {
+				t.Fatalf("maintenance notifications mode = %v, want %v", cfg.Mode, test.want)
+			}
+		})
+	}
+}