fix(限流): 原子化 Redis 限流并支持故障策略

使用 Lua 脚本原子设置计数与过期，修复 TTL 缺失\n支持 fail-open/fail-close 并对优惠码验证启用 fail-close\n新增单元与集成测试覆盖关键分支\n\n测试：go test ./...

backend/internal/middleware/rate_limiter.go

@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"context"
 	"net/http"
 	"time"
 
@@ -8,6 +9,33 @@ import (
 	"github.com/redis/go-redis/v9"
 )
 
+// RateLimitFailureMode Redis 故障策略
+type RateLimitFailureMode int
+
+const (
+	RateLimitFailOpen RateLimitFailureMode = iota
+	RateLimitFailClose
+)
+
+// RateLimitOptions 限流可选配置
+type RateLimitOptions struct {
+	FailureMode RateLimitFailureMode
+}
+
+var rateLimitScript = redis.NewScript(`
+local current = redis.call('INCR', KEYS[1])
+local ttl = redis.call('PTTL', KEYS[1])
+if current == 1 or ttl == -1 then
+  redis.call('PEXPIRE', KEYS[1], ARGV[1])
+end
+return current
+`)
+
+// rateLimitRun 允许测试覆写脚本执行逻辑
+var rateLimitRun = func(ctx context.Context, client *redis.Client, key string, windowMillis int64) (int64, error) {
+	return rateLimitScript.Run(ctx, client, []string{key}, windowMillis).Int64()
+}
+
 // RateLimiter Redis 速率限制器
 type RateLimiter struct {
 	redis  *redis.Client
@@ -27,34 +55,57 @@ func NewRateLimiter(redisClient *redis.Client) *RateLimiter {
 // limit: 时间窗口内最大请求数
 // window: 时间窗口
 func (r *RateLimiter) Limit(key string, limit int, window time.Duration) gin.HandlerFunc {
+	return r.LimitWithOptions(key, limit, window, RateLimitOptions{})
+}
+
+// LimitWithOptions 返回速率限制中间件（带可选配置）
+func (r *RateLimiter) LimitWithOptions(key string, limit int, window time.Duration, opts RateLimitOptions) gin.HandlerFunc {
+	failureMode := opts.FailureMode
+	if failureMode != RateLimitFailClose {
+		failureMode = RateLimitFailOpen
+	}
+
 	return func(c *gin.Context) {
 		ip := c.ClientIP()
 		redisKey := r.prefix + key + ":" + ip
 
 		ctx := c.Request.Context()
 
-		// 使用 INCR 原子操作增加计数
-		count, err := r.redis.Incr(ctx, redisKey).Result()
+		windowMillis := windowTTLMillis(window)
+
+		// 使用 Lua 脚本原子操作增加计数并设置过期
+		count, err := rateLimitRun(ctx, r.redis, redisKey, windowMillis)
 		if err != nil {
+			if failureMode == RateLimitFailClose {
+				abortRateLimit(c)
+				return
+			}
 			// Redis 错误时放行，避免影响正常服务
 			c.Next()
 			return
 		}
 
-		// 首次访问时设置过期时间
-		if count == 1 {
-			r.redis.Expire(ctx, redisKey, window)
-		}
-
 		// 超过限制
 		if count > int64(limit) {
-			c.AbortWithStatusJSON(http.StatusTooManyRequests, gin.H{
-				"error":   "rate limit exceeded",
-				"message": "Too many requests, please try again later",
-			})
+			abortRateLimit(c)
 			return
 		}
 
 		c.Next()
 	}
 }
+
+func windowTTLMillis(window time.Duration) int64 {
+	ttl := window.Milliseconds()
+	if ttl < 1 {
+		return 1
+	}
+	return ttl
+}
+
+func abortRateLimit(c *gin.Context) {
+	c.AbortWithStatusJSON(http.StatusTooManyRequests, gin.H{
+		"error":   "rate limit exceeded",
+		"message": "Too many requests, please try again later",
+	})
+}