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_test.go

@@ -0,0 +1,100 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWindowTTLMillis(t *testing.T) {
+	require.Equal(t, int64(1), windowTTLMillis(500*time.Microsecond))
+	require.Equal(t, int64(1), windowTTLMillis(1500*time.Microsecond))
+	require.Equal(t, int64(2), windowTTLMillis(2500*time.Microsecond))
+}
+
+func TestRateLimiterFailureModes(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rdb := redis.NewClient(&redis.Options{
+		Addr:         "127.0.0.1:1",
+		DialTimeout:  50 * time.Millisecond,
+		ReadTimeout:  50 * time.Millisecond,
+		WriteTimeout: 50 * time.Millisecond,
+	})
+	t.Cleanup(func() {
+		_ = rdb.Close()
+	})
+
+	limiter := NewRateLimiter(rdb)
+
+	failOpenRouter := gin.New()
+	failOpenRouter.Use(limiter.Limit("test", 1, time.Second))
+	failOpenRouter.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder := httptest.NewRecorder()
+	failOpenRouter.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	failCloseRouter := gin.New()
+	failCloseRouter.Use(limiter.LimitWithOptions("test", 1, time.Second, RateLimitOptions{
+		FailureMode: RateLimitFailClose,
+	}))
+	failCloseRouter.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	req = httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder = httptest.NewRecorder()
+	failCloseRouter.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusTooManyRequests, recorder.Code)
+}
+
+func TestRateLimiterSuccessAndLimit(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	originalRun := rateLimitRun
+	counts := []int64{1, 2}
+	callIndex := 0
+	rateLimitRun = func(ctx context.Context, client *redis.Client, key string, windowMillis int64) (int64, error) {
+		if callIndex >= len(counts) {
+			return counts[len(counts)-1], nil
+		}
+		value := counts[callIndex]
+		callIndex++
+		return value, nil
+	}
+	t.Cleanup(func() {
+		rateLimitRun = originalRun
+	})
+
+	limiter := NewRateLimiter(redis.NewClient(&redis.Options{Addr: "127.0.0.1:1"}))
+
+	router := gin.New()
+	router.Use(limiter.Limit("test", 1, time.Second))
+	router.GET("/test", func(c *gin.Context) {
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	req := httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder := httptest.NewRecorder()
+	router.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusOK, recorder.Code)
+
+	req = httptest.NewRequest(http.MethodGet, "/test", nil)
+	req.RemoteAddr = "127.0.0.1:1234"
+	recorder = httptest.NewRecorder()
+	router.ServeHTTP(recorder, req)
+	require.Equal(t, http.StatusTooManyRequests, recorder.Code)
+}