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/channels/qq/adapter.go

@@ -11,8 +11,8 @@ import (
 	"sync"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/message_gateway"
+	"github.com/Rain-kl/Wavelet/pkg/logger"
+	"github.com/Rain-kl/Wavelet/plugins/domain/message_gateway"
 	"github.com/tencent-connect/botgo"
 	"github.com/tencent-connect/botgo/dto"
 	"github.com/tencent-connect/botgo/event"

backend/plugins/domain/message_gateway/channels/qq/adapter_test.go

@@ -7,7 +7,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/message_gateway"
+	"github.com/Rain-kl/Wavelet/plugins/domain/message_gateway"
 )
 
 func TestHandleEvent_DropsNonC2C(t *testing.T) {

backend/plugins/domain/message_gateway/channels/telegram/adapter.go

@@ -12,7 +12,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/message_gateway"
+	"github.com/Rain-kl/Wavelet/plugins/domain/message_gateway"
 	tele "gopkg.in/telebot.v4"
 )
 

backend/plugins/domain/message_gateway/channels/telegram/adapter_test.go

@@ -7,7 +7,7 @@ import (
 	"context"
 	"testing"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/message_gateway"
+	"github.com/Rain-kl/Wavelet/plugins/domain/message_gateway"
 	tele "gopkg.in/telebot.v4"
 )