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/core/app_test.go

@@ -14,8 +14,8 @@ 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/extpoints"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/extpoints"
 )
 
 // appMockDriver is a test driver tracking its start/stop lifecycle.

backend/core/context.go

@@ -10,7 +10,7 @@ import (
 	"sync"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/core/extpoints"
+	"github.com/Rain-kl/Wavelet/core/extpoints"
 )
 
 // Context is the central micro-kernel service bus and runtime lifecycle container.

backend/core/context_test.go

@@ -15,7 +15,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/core"
 )
 
 // Sample services for testing

backend/core/events_test.go

@@ -14,7 +14,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/core"
 )
 
 type UserRegisteredEvent struct {

backend/core/extpoints/extpoints_test.go

@@ -12,8 +12,8 @@ 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/extpoints"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/extpoints"
 )
 
 func TestRouterExtension(t *testing.T) {

backend/core/types.go

@@ -7,7 +7,7 @@ import (
 	"context"
 	"errors"
 
-	"github.com/Rain-kl/Wavelet/backend/core/extpoints"
+	"github.com/Rain-kl/Wavelet/core/extpoints"
 )
 
 // Standard sentinel errors returned by core operations.