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

backend/plugins/domain/message_gateway/push/custom.go

@@ -13,7 +13,7 @@ import (
 	"net/http"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/httppool"
+	"github.com/Rain-kl/Wavelet/pkg/httppool"
 )
 
 func init() {

backend/plugins/domain/message_gateway/push/email.go

@@ -11,7 +11,7 @@ import (
 	"net/smtp"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/util"
+	"github.com/Rain-kl/Wavelet/pkg/util"
 )
 
 func init() {

backend/plugins/domain/message_gateway/push/lark.go

@@ -17,7 +17,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/httppool"
+	"github.com/Rain-kl/Wavelet/pkg/httppool"
 )
 
 func init() {

backend/plugins/domain/message_gateway/push/telegram.go

@@ -12,7 +12,7 @@ import (
 	"net/http"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/httppool"
+	"github.com/Rain-kl/Wavelet/pkg/httppool"
 )
 
 func init() {