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/domain/auth/audit.go

@@ -0,0 +1,38 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package auth
+
+import (
+	"context"
+	"encoding/json"
+
+	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+	"github.com/gin-gonic/gin"
+)
+
+// LogForAudit 将登录鉴权审计日志写入 Logger
+func LogForAudit(ctx context.Context, user *contracts.UserDTO, c *gin.Context) {
+	if user == nil || c == nil {
+		return
+	}
+	auditLog := loginRequiredAuditLog{
+		UserID:     user.ID,
+		Username:   user.Username,
+		ClientIP:   c.ClientIP(),
+		Method:     c.Request.Method,
+		Path:       c.Request.URL.Path,
+		RequestURI: c.Request.RequestURI,
+		UserAgent:  c.Request.UserAgent(),
+		Referer:    c.Request.Referer(),
+	}
+	auditJSON, err := json.Marshal(auditLog)
+	if err != nil {
+		logger.ErrorF(ctx, "[LoginRequiredAudit] marshal failed: %v", err)
+		logger.DebugF(ctx, "[LoginRequiredAudit] %s %d %s", c.ClientIP(), user.ID, user.Username)
+	} else {
+		logger.DebugF(ctx, "[LoginRequiredAudit] %s", auditJSON)
+	}
+}