refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/drivers/driver_http/middlewares_test.go

@@ -0,0 +1,100 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_http
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/testhelper"
+	"github.com/gin-gonic/gin"
+)
+
+func TestCORSMiddleware(t *testing.T) {
+	dbConn, _, cleanup := testhelper.SetupTestEnvironment(t)
+	defer cleanup()
+
+	gin.SetMode(gin.TestMode)
+
+	clearConfigCache := func() {}
+
+	t.Run("missing server_address configuration returns no CORS headers", func(t *testing.T) {
+		clearConfigCache()
+		if err := dbConn.Table("w_system_configs").Where("key = ?", "server_address").Update("value", "").Error; err != nil {
+			t.Fatalf("failed to update config: %v", err)
+		}
+
+		r := gin.New()
+		r.Use(corsMiddleware())
+		r.GET("/test", func(c *gin.Context) {
+			c.String(http.StatusOK, "ok")
+		})
+
+		req, _ := http.NewRequest(http.MethodGet, "/test", nil)
+		req.Header.Set("Origin", "http://attacker.com")
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Header().Get("Access-Control-Allow-Origin") != "" {
+			t.Errorf("expected no Access-Control-Allow-Origin, got %s", w.Header().Get("Access-Control-Allow-Origin"))
+		}
+	})
+
+	t.Run("configured server_address allows exact origin match", func(t *testing.T) {
+		if err := dbConn.Table("w_system_configs").Where("key = ?", "server_address").Update("value", "http://trusted.com").Error; err != nil {
+			t.Fatalf("failed to update config: %v", err)
+		}
+
+		r := gin.New()
+		r.Use(corsMiddleware())
+		r.GET("/test", func(c *gin.Context) {
+			c.String(http.StatusOK, "ok")
+		})
+
+		// Trusted origin
+		req, _ := http.NewRequest(http.MethodGet, "/test", nil)
+		req.Header.Set("Origin", "http://trusted.com")
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Header().Get("Access-Control-Allow-Origin") != "http://trusted.com" {
+			t.Errorf("expected Access-Control-Allow-Origin http://trusted.com, got %s", w.Header().Get("Access-Control-Allow-Origin"))
+		}
+		if w.Header().Get("Access-Control-Allow-Credentials") != "true" {
+			t.Errorf("expected Access-Control-Allow-Credentials true, got %s", w.Header().Get("Access-Control-Allow-Credentials"))
+		}
+
+		// Untrusted origin
+		req, _ = http.NewRequest(http.MethodGet, "/test", nil)
+		req.Header.Set("Origin", "http://attacker.com")
+		w = httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Header().Get("Access-Control-Allow-Origin") != "" {
+			t.Errorf("expected no Access-Control-Allow-Origin for attacker, got %s", w.Header().Get("Access-Control-Allow-Origin"))
+		}
+	})
+
+	t.Run("preflight OPTIONS request responds with 204", func(t *testing.T) {
+		if err := dbConn.Table("w_system_configs").Where("key = ?", "server_address").Update("value", "http://trusted.com").Error; err != nil {
+			t.Fatalf("failed to update config: %v", err)
+		}
+
+		r := gin.New()
+		r.Use(corsMiddleware())
+
+		req, _ := http.NewRequest(http.MethodOptions, "/test", nil)
+		req.Header.Set("Origin", "http://trusted.com")
+		w := httptest.NewRecorder()
+		r.ServeHTTP(w, req)
+
+		if w.Code != http.StatusNoContent {
+			t.Errorf("expected status 204, got %d", w.Code)
+		}
+		if w.Header().Get("Access-Control-Allow-Methods") == "" {
+			t.Error("expected Access-Control-Allow-Methods header")
+		}
+	})
+}