fix: internal reminder路由+user_id支持 — 修复401/404

- Gateway新增 POST/GET/DELETE /api/v1/internal/reminders
- CreateReminderRequest新增UserID字段，JWT为空时回退
- GatewayClient URL改为 /api/v1/internal/reminders

Co-Authored-By: Claude <noreply@anthropic.com>

backend/gateway/internal/router/router.go

@@ -234,6 +234,9 @@ func Setup(r *gin.Engine, hub *ws.Hub, cfg *config.Config, sessionStore *store.S
 	{
 		internal.POST("/notify", notificationHandler.InternalNotify)
 		internal.POST("/proactive-message", chatHandler.HandleProactiveMessage)
+		internal.POST("/reminders", reminderHandler.Create)
+		internal.GET("/reminders", reminderHandler.List)
+		internal.DELETE("/reminders/:id", reminderHandler.Delete)
 	}
 
 	// ========== WebSocket路由 ==========