perf: concurrent ASOS fetches + bulk upsert in commercial poll worker

- Replace sequential Enum.each with Task.async_stream for station I/O
- Replace per-row upsert_surface_observation with batch upsert_surface_observations

lib/microwaveprop/commercial/poll_worker.ex

@@ -67,7 +67,12 @@ defmodule Microwaveprop.Commercial.PollWorker do
     now = DateTime.utc_now()
     start_dt = DateTime.add(now, -3600, :second)
 
-    Enum.each(stations, &fetch_station_weather(&1, start_dt, now))
+    stations
+    |> Task.async_stream(&fetch_station_weather(&1, start_dt, now),
+      timeout: :infinity,
+      max_concurrency: System.schedulers_online()
+    )
+    |> Stream.run()
   end
 
   defp fetch_station_weather(station_code, start_dt, now) do
@@ -90,9 +95,8 @@ defmodule Microwaveprop.Commercial.PollWorker do
   end
 
   defp ingest_asos(station, station_code, rows) do
-    rows
-    |> Enum.filter(& &1.observed_at)
-    |> Enum.each(&Weather.upsert_surface_observation(station, &1))
+    filtered = Enum.filter(rows, & &1.observed_at)
+    Weather.upsert_surface_observations(station, filtered)
 
     Logger.info("Fetched #{length(rows)} ASOS observations for #{station_code}")
   end