A task can exist in the database but not have any log stored on disk yet.
This is now returned as `204 No Content` instead of an internal server
error.
The web interface is also adjusted to cope with this.
Implement task log broadcasting via SocketIO. The logs aren't shown in the
web interface yet, but do arrive there in a Pinia store. That store is
capped at 1000 lines to keep memory requirements low-ish.