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) {