feat(workers): add GridCachePruneWorker

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.

config/config.exs

@@ -98,6 +98,7 @@ config :microwaveprop, Oban,
        {"*/5 * * * *", Microwaveprop.Commercial.PollWorker},
        {"5 * * * *", Microwaveprop.Workers.PropagationGridWorker},
        {"*/15 * * * *", Microwaveprop.Workers.PropagationPruneWorker},
+       {"15 * * * *", Microwaveprop.Workers.GridCachePruneWorker},
        # UWYO publishes the 00Z/12Z Canadian radiosondes ~90 minutes after launch
        {"30 1 * * *", CanadianSoundingFetchWorker},
        {"30 13 * * *", CanadianSoundingFetchWorker},

config/dev.exs

@@ -97,6 +97,7 @@ config :microwaveprop, Oban,
      crontab: [
        {"5 * * * *", Microwaveprop.Workers.PropagationGridWorker},
        {"*/15 * * * *", Microwaveprop.Workers.PropagationPruneWorker},
+       {"15 * * * *", Microwaveprop.Workers.GridCachePruneWorker},
        {"0 8 * * *", Microwaveprop.Workers.SolarIndexWorker}
      ]}
   ]

config/runtime.exs

@@ -205,6 +205,7 @@ if config_env() == :prod do
        # (25 jobs per run, 100/day) into the :gefs queue.
        {"30 5,11,17,23 * * *", Microwaveprop.Workers.GefsFetchWorker},
        {"*/15 * * * *", Microwaveprop.Workers.PropagationPruneWorker},
+       {"15 * * * *", Microwaveprop.Workers.GridCachePruneWorker},
        # The :narr type is virtual: it targets contacts with hrrr_status =
        # :unavailable (pre-2014, missing from the HRRR archive) and dispatches
        # NarrFetchWorker against NCEI. See narr_jobs_for_contact/1.

lib/microwaveprop/workers/grid_cache_prune_worker.ex

@@ -0,0 +1,28 @@
+defmodule Microwaveprop.Workers.GridCachePruneWorker do
+  @moduledoc """
+  Prunes stale rows from the node-local `Microwaveprop.Weather.GridCache` ETS
+  table. Without this cron the cache grew unbounded: every hourly
+  `PropagationGridWorker` run plus peer `broadcast_put` added an entry per
+  forecast hour (~92k points * 19 hours), so long-lived pods eventually OOM'd.
+
+  Runs on the shared `:propagation` queue alongside `PropagationPruneWorker`
+  at a lower priority so the hourly grid chain never waits behind it.
+  """
+
+  use Oban.Worker,
+    queue: :propagation,
+    priority: 5,
+    max_attempts: 3,
+    unique: [period: 300, states: [:available, :scheduled, :executing, :retryable]]
+
+  alias Microwaveprop.Weather.GridCache
+
+  @cutoff_hours 3
+
+  @impl Oban.Worker
+  def perform(%Oban.Job{}) do
+    cutoff = DateTime.add(DateTime.utc_now(), -@cutoff_hours, :hour)
+    _removed = GridCache.prune_older_than(cutoff)
+    :ok
+  end
+end

test/microwaveprop/workers/grid_cache_prune_worker_test.exs

@@ -0,0 +1,40 @@
+defmodule Microwaveprop.Workers.GridCachePruneWorkerTest do
+  use ExUnit.Case, async: false
+
+  alias Microwaveprop.Weather.GridCache
+  alias Microwaveprop.Workers.GridCachePruneWorker
+
+  setup do
+    GridCache.clear()
+    :ok
+  end
+
+  describe "perform/1" do
+    test "removes cache entries with valid_time older than 3 hours" do
+      now = DateTime.utc_now()
+      old = now |> DateTime.add(-3, :hour) |> DateTime.add(-1, :second) |> DateTime.truncate(:second)
+      fresh = now |> DateTime.add(-30, :minute) |> DateTime.truncate(:second)
+      future = now |> DateTime.add(6, :hour) |> DateTime.truncate(:second)
+
+      GridCache.put(old, [%{lat: 32.0, lon: -97.0, temperature: 10.0}])
+      GridCache.put(fresh, [%{lat: 32.0, lon: -97.0, temperature: 20.0}])
+      GridCache.put(future, [%{lat: 32.0, lon: -97.0, temperature: 30.0}])
+
+      assert :ok = GridCachePruneWorker.perform(%Oban.Job{})
+
+      assert GridCache.fetch(old) == :miss
+      assert {:ok, [%{temperature: 20.0}]} = GridCache.fetch(fresh)
+      assert {:ok, [%{temperature: 30.0}]} = GridCache.fetch(future)
+    end
+
+    test "is a no-op when nothing is stale" do
+      fresh =
+        DateTime.utc_now() |> DateTime.add(-30, :minute) |> DateTime.truncate(:second)
+
+      GridCache.put(fresh, [%{lat: 32.0, lon: -97.0, temperature: 42.0}])
+
+      assert :ok = GridCachePruneWorker.perform(%Oban.Job{})
+      assert {:ok, [%{temperature: 42.0}]} = GridCache.fetch(fresh)
+    end
+  end
+end