fix: 修复19个Bug (P0-P3) — 持续性调试第7轮发现的问题

P0 (5): crypto/rand session ID, TTS fallback可达性, goroutine defer recover, adminAuth前缀修正
P1 (5): 普通用户密码验证, context传递, priority clamp, 超时重试, 自主思考速率限制
P2 (4): Briefing AI降级, 前端消息类型渲染, Docker Compose补全, PWA 192图标
P3 (5): goroutine错误处理, .gitignore完善, reminder created_at, voice Dockerfile, Go版本更新

backend/gateway/internal/handler/session_handler.go

@@ -1,6 +1,7 @@
 package handler
 
 import (
+	"crypto/rand"
 	"encoding/json"
 	"fmt"
 	"log"
@@ -576,8 +577,15 @@ func (h *SessionHandler) exportTXT(c *gin.Context, session *store.Session, messa
 func randomID(n int) string {
 	const letters = "abcdefghijklmnopqrstuvwxyz0123456789"
 	b := make([]byte, n)
+	if _, err := rand.Read(b); err != nil {
+		// fallback to deterministic IDs only if crypto/rand fails
+		for i := range b {
+			b[i] = letters[i%len(letters)]
+		}
+		return string(b)
+	}
 	for i := range b {
-		b[i] = letters[i%len(letters)]
+		b[i] = letters[int(b[i])%len(letters)]
 	}
 	return string(b)
 }