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/drivers/driver_asynq_worker/utils_test.go

@@ -0,0 +1,59 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_asynq_worker
+
+import (
+	"testing"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+	"github.com/redis/go-redis/v9/maintnotifications"
+)
+
+func TestMaintNotificationsConfig(t *testing.T) {
+	tests := []struct {
+		name     string
+		enabled  bool
+		wantMode maintnotifications.Mode
+	}{
+		{
+			name:     "disabled mode when flag is false",
+			enabled:  false,
+			wantMode: maintnotifications.ModeDisabled,
+		},
+		{
+			name:     "auto mode when flag is true",
+			enabled:  true,
+			wantMode: maintnotifications.ModeAuto,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := maintNotificationsConfig(tt.enabled)
+			if cfg == nil {
+				t.Fatalf("maintNotificationsConfig(%v) = nil, want non-nil", tt.enabled)
+			}
+			if cfg.Mode != tt.wantMode {
+				t.Fatalf("maintNotificationsConfig(%v).Mode = %v, want %v", tt.enabled, cfg.Mode, tt.wantMode)
+			}
+		})
+	}
+}
+
+func TestPrefixedQueue(t *testing.T) {
+	oldPrefix := config.Config.Redis.KeyPrefix
+	defer func() {
+		config.Config.Redis.KeyPrefix = oldPrefix
+	}()
+
+	config.Config.Redis.KeyPrefix = "test:"
+	if got := PrefixedQueue("default"); got != "test:default" {
+		t.Fatalf("PrefixedQueue() = %q, want %q", got, "test:default")
+	}
+
+	config.Config.Redis.KeyPrefix = ""
+	if got := PrefixedQueue("default"); got != "default" {
+		t.Fatalf("PrefixedQueue() = %q, want %q", got, "default")
+	}
+}