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/pkg/testhelper/gin.go

@@ -4,7 +4,7 @@
 package testhelper
 
 import (
-	"github.com/Rain-kl/Wavelet/backend/pkg/response"
+	"github.com/Rain-kl/Wavelet/pkg/response"
 	"github.com/gin-gonic/gin"
 )
 

backend/pkg/testhelper/test_helper.go

@@ -10,8 +10,8 @@ import (
 	"testing"
 	"time"
 
-	cachepkg "github.com/Rain-kl/Wavelet/backend/plugins/infra/cache"
-	db "github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	cachepkg "github.com/Rain-kl/Wavelet/plugins/infra/cache"
+	db "github.com/Rain-kl/Wavelet/plugins/infra/database"
 	"github.com/alicebob/miniredis/v2"
 	"github.com/glebarez/sqlite"
 	"github.com/redis/go-redis/v9"