feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/internal/handler/middleware/logging.go

@@ -0,0 +1,40 @@
+package middleware
+
+import (
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"go.uber.org/zap"
+)
+
+// Logging 日志中间件
+func Logging(logger *zap.Logger) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		start := time.Now()
+		path := c.Request.URL.Path
+		query := c.Request.URL.RawQuery
+
+		requestID, _ := c.Get(RequestIDKey)
+		logger.Info("请求开始",
+			zap.String("method", c.Request.Method),
+			zap.String("path", path),
+			zap.String("query", query),
+			zap.String("client_ip", c.ClientIP()),
+			zap.Any("request_id", requestID),
+		)
+
+		c.Next()
+
+		latency := time.Since(start)
+		statusCode := c.Writer.Status()
+
+		logger.Info("请求结束",
+			zap.Int("status", statusCode),
+			zap.String("method", c.Request.Method),
+			zap.String("path", path),
+			zap.Duration("latency", latency),
+			zap.Int("body_size", c.Writer.Size()),
+			zap.Any("request_id", requestID),
+		)
+	}
+}