feat(backend): Go 服务骨架（配置、日志、数据库、中间件、Wire）

- config: Viper YAML 配置 + 环境变量覆盖
- logs: zap 结构化日志 + trace_id 链路追踪 + 敏感信息脱敏
- db: PostgreSQL (GORM) + Redis (go-redis) 连接管理
- middleware: Trace/Logger/CORS/Recovery 四层中间件
- provider: Wire 编译时依赖注入
- main.go: 优雅启动/关闭 HTTP 服务

Made-with: Cursor

backend/go-service/pkg/middleware/cors.go

@@ -0,0 +1,26 @@
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// CORS 跨域中间件
+// 允许前端跨域访问后端 API
+func CORS() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Header("Access-Control-Allow-Origin", "*")
+		c.Header("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS, PATCH")
+		c.Header("Access-Control-Allow-Headers", "Origin, Content-Type, Authorization, X-Request-ID")
+		c.Header("Access-Control-Expose-Headers", "X-Request-ID")
+		c.Header("Access-Control-Max-Age", "86400")
+
+		if c.Request.Method == http.MethodOptions {
+			c.AbortWithStatus(http.StatusNoContent)
+			return
+		}
+
+		c.Next()
+	}
+}