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/upload/storage/access_state.go

@@ -9,9 +9,9 @@ import (
 	"sync"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/shared"
-	"github.com/Rain-kl/Wavelet/backend/plugins/drivers/driver_asynq_worker"
-	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage/objectstore"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/shared"
+	"github.com/Rain-kl/Wavelet/plugins/drivers/driver_asynq_worker"
+	"github.com/Rain-kl/Wavelet/plugins/infra/storage/objectstore"
 )
 
 // MigrationAccessState captures cached migration maintenance state.

backend/plugins/domain/upload/storage/migration.go

@@ -10,8 +10,8 @@ import (
 	"fmt"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/plugins/drivers/driver_asynq_worker"
-	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage/objectstore"
+	"github.com/Rain-kl/Wavelet/plugins/drivers/driver_asynq_worker"
+	"github.com/Rain-kl/Wavelet/plugins/infra/storage/objectstore"
 )
 
 // StorageMigrationTask is the Asynq task name for storage migration.

backend/plugins/domain/upload/storage/storage_ops.go

@@ -6,9 +6,9 @@ package storage
 import (
 	"context"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/models"
-	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage/objectstore"
+	"github.com/Rain-kl/Wavelet/pkg/logger"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/models"
+	"github.com/Rain-kl/Wavelet/plugins/infra/storage/objectstore"
 )
 
 // ReadOnly checks if the storage system is in read-only maintenance mode.