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/user/handlers.go

@@ -12,10 +12,10 @@ import (
 	"strconv"
 	"time"
 
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/pkg/response"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/pkg/response"
 	"github.com/gin-contrib/sessions"
 	"github.com/gin-gonic/gin"
 )

backend/plugins/domain/user/models.go

@@ -10,7 +10,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/util"
+	"github.com/Rain-kl/Wavelet/pkg/util"
 )
 
 // AccessToken 个人访问令牌实体

backend/plugins/domain/user/plugin.go

@@ -8,9 +8,9 @@ import (
 	"context"
 	"embed"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/core/extpoints"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/core/extpoints"
 	"github.com/gin-gonic/gin"
 	"github.com/hibiken/asynq"
 )

backend/plugins/domain/user/plugin_test.go

@@ -13,10 +13,10 @@ import (
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/user"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/plugins/domain/user"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 )
 
 func setupTestDB(t *testing.T) *gorm.DB {

backend/plugins/domain/user/repository.go

@@ -9,8 +9,8 @@ import (
 
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/util"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/Rain-kl/Wavelet/pkg/util"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 	"gorm.io/gorm"
 )
 

backend/plugins/domain/user/service.go

@@ -11,11 +11,11 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/pkg/idgen"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/pkg/idgen"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 
-	pkgu "github.com/Rain-kl/Wavelet/backend/pkg/util"
+	pkgu "github.com/Rain-kl/Wavelet/pkg/util"
 )
 
 func toUserDTO(u *User) *contracts.UserDTO {