Add weather data schema, context, sounding params, and IEM ingestion

Store surface observations (ASOS) and upper-air soundings (RAOB) alongside
QSOs for atmospheric propagation correlation. Three new tables: weather_stations,
surface_observations, and soundings with JSONB profiles and pre-computed derived
parameters (refractivity, gradients, duct detection, stability indices).

Includes IEM API client for historical data import and import script seeded
with 95 ASOS + 9 sounding stations from PropCast coverage area.

test/microwaveprop/weather/station_test.exs

@@ -0,0 +1,71 @@
+defmodule Microwaveprop.Weather.StationTest do
+  use Microwaveprop.DataCase, async: true
+
+  alias Microwaveprop.Weather.Station
+
+  @valid_attrs %{
+    station_code: "KDFW",
+    station_type: "asos",
+    name: "Dallas/Fort Worth International",
+    lat: 32.8998,
+    lon: -97.0403,
+    elevation_m: 171.0,
+    state: "TX"
+  }
+
+  describe "changeset/2" do
+    test "valid attributes" do
+      changeset = Station.changeset(%Station{}, @valid_attrs)
+      assert changeset.valid?
+    end
+
+    test "valid sounding station with wmo_number" do
+      attrs = %{
+        station_code: "FWD",
+        station_type: "sounding",
+        wmo_number: 72_249,
+        name: "Fort Worth",
+        lat: 32.83,
+        lon: -97.30
+      }
+
+      changeset = Station.changeset(%Station{}, attrs)
+      assert changeset.valid?
+    end
+
+    test "requires station_code, station_type, name, lat, lon" do
+      changeset = Station.changeset(%Station{}, %{})
+
+      assert %{
+               station_code: ["can't be blank"],
+               station_type: ["can't be blank"],
+               name: ["can't be blank"],
+               lat: ["can't be blank"],
+               lon: ["can't be blank"]
+             } = errors_on(changeset)
+    end
+
+    test "validates station_type inclusion" do
+      changeset = Station.changeset(%Station{}, %{@valid_attrs | station_type: "radar"})
+      assert %{station_type: ["is invalid"]} = errors_on(changeset)
+    end
+
+    test "persists to the database" do
+      changeset = Station.changeset(%Station{}, @valid_attrs)
+      assert {:ok, station} = Repo.insert(changeset)
+      assert station.station_code == "KDFW"
+      assert station.station_type == "asos"
+      assert station.lat == 32.8998
+      assert station.lon == -97.0403
+    end
+
+    test "enforces unique station_code + station_type" do
+      changeset = Station.changeset(%Station{}, @valid_attrs)
+      assert {:ok, _} = Repo.insert(changeset)
+
+      duplicate = Station.changeset(%Station{}, @valid_attrs)
+      assert {:error, changeset} = Repo.insert(duplicate)
+      assert %{station_code: ["has already been taken"]} = errors_on(changeset)
+    end
+  end
+end