feat(weather): derive hrrr_profiles scalars from existing profile JSONB

The Rust hrrr_points worker now persists surface_refractivity /
min_refractivity_gradient / ducting_detected at upsert time (previous
commit), but the ~80k rows written before that fix have the scalar
columns NULL even though the pressure-level `profile` JSONB is
populated. Re-fetching them would re-pull tens of GB of HRRR grids
we already parsed.

Weather.backfill_hrrr_scalars/1 streams rows with
`surface_refractivity IS NULL`, runs the existing
SoundingParams.derive/1 against the stored profile, and writes the
scalars back — no network, no GRIB decode. Batched (500 rows per
tick, 1000 tick cap) to keep transactions short on the Turing Pi 2
PG node.

Meant to be run once via `bin/microwaveprop rpc` after deploy.
Idempotent; the WHERE clause excludes already-backfilled rows.

lib/microwaveprop/weather.ex

@@ -385,6 +385,75 @@ defmodule Microwaveprop.Weather do
     {:ok, n}
   end
 
+  @doc """
+  Derive `surface_refractivity` / `min_refractivity_gradient` /
+  `ducting_detected` for every `hrrr_profiles` row where those columns
+  are NULL but the pressure-level `profile` JSONB is populated.
+  Required one-off after the Rust hrrr_points worker shipped without
+  deriving these scalars — existing rows have the raw levels but the
+  DB scalars are NULL. Idempotent; rows with scalars already set are
+  skipped by the WHERE clause.
+
+  Batches by `limit` rows per pass to keep transactions short on the
+  Turing Pi 2 Postgres node. Returns the total number of rows advanced.
+  """
+  @spec backfill_hrrr_scalars(keyword()) :: non_neg_integer()
+  def backfill_hrrr_scalars(opts \\ []) do
+    batch = Keyword.get(opts, :batch_size, 500)
+    max_batches = Keyword.get(opts, :max_batches, 1_000)
+
+    Enum.reduce_while(1..max_batches, 0, fn _, total ->
+      case backfill_hrrr_batch(batch) do
+        0 -> {:halt, total}
+        n -> {:cont, total + n}
+      end
+    end)
+  end
+
+  defp backfill_hrrr_batch(limit) do
+    rows =
+      HrrrProfile
+      |> where([h], is_nil(h.surface_refractivity))
+      |> limit(^limit)
+      |> select([h], %{id: h.id, profile: h.profile})
+      |> Repo.all()
+
+    updates =
+      Enum.flat_map(rows, fn %{id: id, profile: profile} ->
+        case SoundingParams.derive(profile || []) do
+          %{} = derived ->
+            [
+              %{
+                id: id,
+                surface_refractivity: Map.get(derived, :surface_refractivity),
+                min_refractivity_gradient: Map.get(derived, :min_refractivity_gradient),
+                ducting_detected: Map.get(derived, :ducting_detected, false),
+                duct_characteristics: Map.get(derived, :duct_characteristics)
+              }
+            ]
+
+          _ ->
+            []
+        end
+      end)
+
+    Enum.each(updates, fn u ->
+      HrrrProfile
+      |> where([h], h.id == ^u.id)
+      |> Repo.update_all(
+        set: [
+          surface_refractivity: u.surface_refractivity,
+          min_refractivity_gradient: u.min_refractivity_gradient,
+          ducting_detected: u.ducting_detected,
+          duct_characteristics: u.duct_characteristics,
+          updated_at: DateTime.truncate(DateTime.utc_now(), :second)
+        ]
+      )
+    end)
+
+    length(updates)
+  end
+
   @doc """
   True if the station already has at least one surface observation
   anywhere within the given UTC date. Used by the `asos_day` worker