Add a propagation_run_timings table so the wall-clock duration of each
(run_time, forecast_hour) step is queryable long after the run is over.
Keyed by (run_time, forecast_hour) with a status column that captures
whether the step succeeded or bailed out, and an error string on
failure.
PropagationGridWorker stamps every step (ok and failed) via
Propagation.record_run_timing/1. Timing inserts are wrapped in rescue +
changeset-error handling so the instrumentation can never brick the
chain.
