refactor(layout): relocate go.mod to backend/ and clean import paths to module root

- Relocated go.mod and go.sum into backend/ root directory
- Stripped redundant backend/ segments from all Go imports (github.com/Rain-kl/Wavelet/...)
- Unified Makefile, swagger, and build-test to execute in backend/ module context
- Ensured 100% build-test, code-check, format, and swagger pass

.agents/skills/push-notification/SKILL.md

@@ -39,7 +39,7 @@ import (
 	"time"
 
 	"github.com/Rain-kl/Wavelet/internal/apps/admin/push"
-	"github.com/Rain-kl/Wavelet/backend/pkg/listener"
+	"github.com/Rain-kl/Wavelet/pkg/listener"
 )
 
 var NewUserRegistered = push.EventMetadata{
@@ -84,7 +84,7 @@ func Register() {
 在业务逻辑完成处（如 `internal/apps/user/routers.go`）仅 import `internal/listener` 并发射事件：
 
 ```go
-import "github.com/Rain-kl/Wavelet/backend/pkg/listener"
+import "github.com/Rain-kl/Wavelet/pkg/listener"
 
 func Register(c *gin.Context) {
 	// ... 注册成功逻辑 ...