fix: propagation overlay broken by iolist→binary crash in encode_binary

Two regressions from the recent performance commits:

1. ScoresController.encode_binary/1 (9cce257d): Enum.reduce produces
   iolists, but <<lats::binary>> requires actual binaries. Caused HTTP
   500 on every /scores/cells request when score files exist on disk,
   making the entire propagation heatmap overlay invisible.

2. ScoreCache.valid_times/1 (15f4175c): Pipe syntax feeds @table as
   the first argument to :ets.foldl/3, creating a 4-argument call
   that crashes at runtime.

lib/microwaveprop/propagation/score_cache.ex

@@ -149,8 +149,7 @@ defmodule Microwaveprop.Propagation.ScoreCache do
   @doc "Returns the sorted list of valid_times cached for `band_mhz`."
   @spec valid_times(non_neg_integer()) :: [DateTime.t()]
   def valid_times(band_mhz) do
-    @table
-    |> :ets.foldl(
+    :ets.foldl(
       fn
         {{^band_mhz, vt}, _}, acc -> [vt | acc]
         _, acc -> acc

lib/microwaveprop_web/controllers/scores_controller.ex

@@ -110,7 +110,8 @@ defmodule MicrowavepropWeb.ScoresController do
          [<<clamp_score(s.score)::8>> | scs_acc]}
       end)
 
-    <<header::binary, lats::binary, lons::binary, score_bytes::binary>>
+    <<header::binary, IO.iodata_to_binary(lats)::binary,
+      IO.iodata_to_binary(lons)::binary, IO.iodata_to_binary(score_bytes)::binary>>
   end
 
   defp clamp_score(s) when is_integer(s) and s < 0, do: 0