fix: auto-register OnConflict 移除 agent_version（防旧Agent空字段覆盖）

backend/internal/api/handlers/agent.go

@@ -174,7 +174,7 @@ func (h *AgentHandler) ReceiveReport(c *gin.Context) {
 			Columns: []clause.Column{{Name: "hostname"}},
 			DoUpdates: clause.AssignmentColumns([]string{
 				"ip", "os", "cpu", "memory", "disk",
-				"status", "agent_status", "agent_version",
+				"status", "agent_status",
 				"cpu_usage", "memory_usage", "disk_usage",
 				"updated_at",
 			}),