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/meta_test.go

@@ -0,0 +1,98 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_asynq_worker
+
+import (
+	"sync"
+	"testing"
+)
+
+func resetDispatchableTasks() {
+	dispatchableTasksMutex.Lock()
+	defer dispatchableTasksMutex.Unlock()
+	dispatchableTasks = nil
+}
+
+func TestRegisterTaskMeta_ThreadSafe(t *testing.T) {
+	resetDispatchableTasks()
+	defer resetDispatchableTasks()
+
+	var wg sync.WaitGroup
+	workers := 20
+	iterations := 100
+
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func(id int) {
+			defer wg.Done()
+			for j := 0; j < iterations; j++ {
+				RegisterTaskMeta(TaskMeta{
+					Type:      "task_type_a",
+					AsynqTask: "asynq_task_a",
+					Name:      "Task A",
+				})
+				RegisterTaskMeta(TaskMeta{
+					Type:      "task_type_b",
+					AsynqTask: "asynq_task_b",
+					Name:      "Task B",
+				})
+			}
+		}(i)
+	}
+
+	wg.Wait()
+
+	tasks := GetDispatchableTasks()
+	if len(tasks) != 2 {
+		t.Fatalf("expected 2 unique tasks, got %d", len(tasks))
+	}
+}
+
+func TestGetTaskMeta_Isolation(t *testing.T) {
+	resetDispatchableTasks()
+	defer resetDispatchableTasks()
+
+	RegisterTaskMeta(TaskMeta{
+		Type:      "task_isolated",
+		AsynqTask: "asynq_isolated",
+		Name:      "Original Name",
+	})
+
+	meta := GetTaskMeta("task_isolated")
+	if meta == nil {
+		t.Fatal("expected task to be found")
+	}
+
+	// Modify returned struct
+	meta.Name = "Modified Name"
+
+	// Fetch again and verify original data is unaffected
+	meta2 := GetTaskMeta("task_isolated")
+	if meta2.Name != "Original Name" {
+		t.Fatalf("expected name to be 'Original Name', got %s", meta2.Name)
+	}
+}
+
+func TestGetTaskMetaByAsynqTask_Isolation(t *testing.T) {
+	resetDispatchableTasks()
+	defer resetDispatchableTasks()
+
+	RegisterTaskMeta(TaskMeta{
+		Type:      "task_isolated",
+		AsynqTask: "asynq_isolated",
+		Name:      "Original Name",
+	})
+
+	meta := GetTaskMetaByAsynqTask("asynq_isolated")
+	if meta == nil {
+		t.Fatal("expected task to be found")
+	}
+
+	meta.Name = "Modified Name"
+
+	meta2 := GetTaskMetaByAsynqTask("asynq_isolated")
+	if meta2.Name != "Original Name" {
+		t.Fatalf("expected name to be 'Original Name', got %s", meta2.Name)
+	}
+}