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/cache/plugin.go

@@ -11,10 +11,10 @@ import (
 	"sync"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/pkg/cache/ram"
-	"github.com/Rain-kl/Wavelet/backend/pkg/util"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/pkg/cache/ram"
+	"github.com/Rain-kl/Wavelet/pkg/util"
 	"github.com/redis/go-redis/v9"
 )
 

backend/plugins/infra/cache/plugin_test.go

@@ -13,9 +13,9 @@ 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/cache"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/plugins/infra/cache"
 )
 
 func TestCachePluginOperations(t *testing.T) {

backend/plugins/infra/cache/redis.go

@@ -12,7 +12,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+	"github.com/Rain-kl/Wavelet/pkg/config"
 	"github.com/redis/go-redis/extra/redisotel/v9"
 	"github.com/redis/go-redis/v9"
 	"github.com/redis/go-redis/v9/maintnotifications"