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/infra/storage/diskcache/cache.go

@@ -10,8 +10,8 @@ import (
 	"sync"
 	"time"
 
-	pkgcache "github.com/Rain-kl/Wavelet/backend/pkg/cache/disk"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	pkgcache "github.com/Rain-kl/Wavelet/pkg/cache/disk"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 )
 
 // Status represents the runtime cache statistics.

backend/plugins/infra/storage/diskcache/cache_test.go

@@ -8,8 +8,8 @@ import (
 	"os"
 	"testing"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/testhelper"
-	cache "github.com/Rain-kl/Wavelet/backend/plugins/infra/cache"
+	"github.com/Rain-kl/Wavelet/pkg/testhelper"
+	cache "github.com/Rain-kl/Wavelet/plugins/infra/cache"
 )
 
 func TestDiskCacheReloadConfig(t *testing.T) {

backend/plugins/infra/storage/objectstore/config.go

@@ -12,7 +12,7 @@ import (
 	"strings"
 	"time"
 
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 
 	"gorm.io/gorm"
 )

backend/plugins/infra/storage/objectstore/http.go

@@ -10,7 +10,7 @@ import (
 	"net/url"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/httppool"
+	"github.com/Rain-kl/Wavelet/pkg/httppool"
 )
 
 func getHTTPObject(ctx context.Context, baseURL, key string) (*Object, error) {

backend/plugins/infra/storage/objectstore/storage.go

@@ -13,10 +13,10 @@ import (
 	"sync"
 	"time"
 
-	cache "github.com/Rain-kl/Wavelet/backend/plugins/infra/cache"
-	database "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	cache "github.com/Rain-kl/Wavelet/plugins/infra/cache"
+	database "github.com/Rain-kl/Wavelet/plugins/infra/database"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/util"
+	"github.com/Rain-kl/Wavelet/pkg/util"
 	"gorm.io/gorm"
 )
 

backend/plugins/infra/storage/objectstore/storage_test.go

@@ -11,7 +11,7 @@ import (
 	"testing"
 	"time"
 
-	cache "github.com/Rain-kl/Wavelet/backend/plugins/infra/cache"
+	cache "github.com/Rain-kl/Wavelet/plugins/infra/cache"
 	"github.com/alicebob/miniredis/v2"
 	"github.com/redis/go-redis/v9"
 )

backend/plugins/infra/storage/objectstore/webdav.go

@@ -11,7 +11,7 @@ import (
 	"path"
 	"strings"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/httppool"
+	"github.com/Rain-kl/Wavelet/pkg/httppool"
 	"github.com/studio-b12/gowebdav"
 )
 

backend/plugins/infra/storage/plugin.go

@@ -9,11 +9,11 @@ import (
 	"fmt"
 	"io"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/plugins/domain/upload/ingest"
-	uploadmodels "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/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/plugins/domain/upload/ingest"
+	uploadmodels "github.com/Rain-kl/Wavelet/plugins/domain/upload/models"
+	"github.com/Rain-kl/Wavelet/plugins/infra/storage/objectstore"
 )
 
 // Option configures the storage plugin.

backend/plugins/infra/storage/plugin_test.go

@@ -14,10 +14,10 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage"
-	"github.com/Rain-kl/Wavelet/backend/plugins/infra/storage/objectstore"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/plugins/infra/storage"
+	"github.com/Rain-kl/Wavelet/plugins/infra/storage/objectstore"
 )
 
 type mockStorageBackend struct {