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/briefing_handler.go

@@ -221,6 +221,9 @@ func (h *BriefingHandler) GenerateDailyBriefing(userID string) (*store.Briefing,
 	if err != nil {
 		log.Printf("[briefing] AI 摘要生成失败 (降级): %v", err)
 		summary = h.buildFallbackSummary(briefing)
+		briefing.SummarySource = "fallback"
+	} else {
+		briefing.SummarySource = "ai"
 	}
 	briefing.Summary = summary