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/chat_handler.go

@@ -3,6 +3,8 @@ package handler
 import (
 	"bufio"
 	"bytes"
+	"crypto/rand"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -433,12 +435,14 @@ func generateID() string {
 }
 
 func randomStr(n int) string {
-	const letters = "abcdefghijklmnopqrstuvwxyz0123456789"
 	b := make([]byte, n)
-	for i := range b {
-		b[i] = letters[time.Now().UnixNano()%int64(len(letters))]
+	if _, err := rand.Read(b); err != nil {
+		// fallback: deterministic but hard to predict
+		for i := range b {
+			b[i] = byte(time.Now().UnixNano()%256)
+		}
 	}
-	return string(b)
+	return hex.EncodeToString(b)[:n]
 }
 
 // parseMultiMessage 检测并解析多消息格式