The Microwaveprop.Weather.GridCache ETS table grew unbounded — each
hourly PropagationGridWorker run plus peer broadcast_put added ~92k
points per forecast hour, so long-lived pods were strong OOM
candidates. GridCache already exposed prune_older_than/1 but nothing
called it.
Adds an Oban worker on the :propagation queue (lower priority than
the grid chain) that prunes entries older than 3 hours. Scheduled
hourly at :15 in dev, prod, and the base crontab.
