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/database/clickhouse.go

@@ -16,7 +16,7 @@ import (
 
 	"github.com/ClickHouse/clickhouse-go/v2"
 	"github.com/ClickHouse/clickhouse-go/v2/lib/driver"
-	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+	"github.com/Rain-kl/Wavelet/pkg/config"
 	"go.opentelemetry.io/otel/attribute"
 	clickhouseDriver "gorm.io/driver/clickhouse"
 	"gorm.io/gorm"

backend/plugins/infra/database/plugin.go

@@ -7,8 +7,8 @@ package database
 import (
 	"context"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
 	"gorm.io/gorm"
 )
 

backend/plugins/infra/database/plugin_test.go

@@ -13,9 +13,9 @@ import (
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
 
-	"github.com/Rain-kl/Wavelet/backend/core"
-	"github.com/Rain-kl/Wavelet/backend/core/contracts"
-	"github.com/Rain-kl/Wavelet/backend/plugins/infra/database"
+	"github.com/Rain-kl/Wavelet/core"
+	"github.com/Rain-kl/Wavelet/core/contracts"
+	"github.com/Rain-kl/Wavelet/plugins/infra/database"
 )
 
 type Item struct {

backend/plugins/infra/database/postgres.go

@@ -12,7 +12,7 @@ import (
 	"strconv"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+	"github.com/Rain-kl/Wavelet/pkg/config"
 	"github.com/glebarez/sqlite"
 	"go.opentelemetry.io/otel/attribute"
 	"gorm.io/driver/postgres"

backend/plugins/infra/database/postgres_logger.go

@@ -11,7 +11,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+	"github.com/Rain-kl/Wavelet/pkg/logger"
 	"gorm.io/gorm"
 	gormLogger "gorm.io/gorm/logger"
 )