fix(propagation): fire grid cron every 15 min to catch fresh HRRR

NOAA publish latency for HRRR pressure-level f18 straddles ~50-65 min
after cycle hour. The HH:05 cron frequently probes a (now-1h) cycle
that's not yet on S3 and falls back to (now-2h). Until today the next
re-attempt was a full hour later, leaving weather visibly stale.

Quadrupling the schedule lets pick_run_time/1 upgrade to the freshest
cycle within 15 min of it publishing. seed_with_analysis is idempotent
(on_conflict: :nothing), so re-fires against an already-seeded run_time
are a single cheap INSERT batch.

config/runtime.exs

@@ -237,14 +237,16 @@ if config_env() == :prod do
      crontab: [
        {"0 8 * * *", Microwaveprop.Workers.SolarIndexWorker},
        {"*/5 * * * *", Microwaveprop.Commercial.PollWorker},
-       # Hourly at :05. With scores written as binary files (not
-       # Postgres) and HRRR grid profile persistence removed, a
-       # full f00-f18 chain runs in ~45-60 min, so hourly fits.
-       # If one chain slips past 60 min the :propagation queue's
-       # concurrency-of-2 lets the next chain interleave; files
-       # are keyed by (band, valid_time) so last-writer-wins gives
-       # newer analysis data naturally.
-       {"5 * * * *", Microwaveprop.Workers.PropagationGridWorker},
+       # Every 15 min. NOAA's HRRR pressure-level f18 publish latency
+       # straddles ~50-65 min after cycle hour, so an HH:05 fire
+       # frequently has to fall back from (now-1h) to (now-2h). A
+       # 4×/hour cadence lets pick_run_time/1 upgrade from fallback
+       # to the freshest cycle within 15 min of it becoming available
+       # instead of waiting for the next HH:05. Re-seeds are cheap:
+       # GridTaskEnqueuer.seed_with_analysis uses on_conflict: :nothing
+       # so duplicate fires for an already-seeded run_time are a single
+       # idempotent INSERT batch.
+       {"5,20,35,50 * * * *", Microwaveprop.Workers.PropagationGridWorker},
        # GEFS runs publish ~3-4h after the 00/06/12/18Z cycle; seed the
        # Day 2-7 outlook 5h after each run to stay safely past NOMADS'
        # publication lag. Seeder enqueues f024-f168 at 6-hour cadence