fix: 第3轮调试Bug修复 (P1: 弱随机+crypto/rand, Knowledge键对齐; P2: 简报created_at, CORS+安全头)

backend/gateway/internal/handler/briefing_handler.go

@@ -162,13 +162,14 @@ func (h *BriefingHandler) GenerateDailyBriefing(userID string) (*store.Briefing,
 	today := time.Now().Format("2006-01-02")
 
 	briefing := &store.Briefing{
-		ID:      "brief_" + generateID(),
-		UserID:  userID,
-		Date:    today,
-		Status:  "pending",
-		Weather: &store.WeatherData{},
-		News:    []store.NewsItem{},
+		ID:        "brief_" + generateID(),
+		UserID:    userID,
+		Date:      today,
+		Status:    "pending",
+		Weather:   &store.WeatherData{},
+		News:      []store.NewsItem{},
 		Reminders: []store.BriefReminder{},
+		CreatedAt: time.Now(),
 	}
 
 	// 1. 获取天气数据