FreshnessMonitor's comment claimed 'Oban unique constraint on
PropagationGridWorker prevents duplicates' — but the worker declared
no unique: option. During a long outage the monitor's 5-min stale-check
tick would pile up identical jobs (a 2h outage = 24 stacked jobs, each
doing the full f00-f18 HRRR chain).
Put the dedup on the worker (vs. the monitor's insert call) so anything
enqueuing it benefits — FreshnessMonitor, the hourly cron, or a manual
mix propagation_grid run.
unique: [period: 3600, states: [:available, :scheduled, :executing,
:retryable]] aligns with the hourly cron cadence. The seed job args
(%{}) collapse with themselves while chain-step jobs keep distinct
run_time/forecast_hour and remain enqueuable.
10 characterization tests: fresh vs stale thresholds (>120 min),
boundary behavior, empty scores dir, supervised start, non-dedup of
repeated stale ticks, :ignore when disabled via config.
