perf: fix medium-severity N+1 and inefficiency patterns

- pending_edit_for_user: add preload for [:user, :contact] to avoid
  lazy-loaded N+1 when templates access those associations
- ScoresController.encode_binary: combine 3 Enum.map passes into a
  single Enum.reduce, halving list traversals for score binary encoding
- find_duplicate_contact: push station-pair/grid matching into SQL
  WHERE clause so dedup uses the DB index instead of loading all
  matching rows + Enum.find in Elixir

lib/microwaveprop/radio.ex

@@ -757,25 +757,18 @@ defmodule Microwaveprop.Radio do
     min_ts = DateTime.add(ts, -@dedup_window_seconds, :second)
     max_ts = DateTime.add(ts, @dedup_window_seconds, :second)
 
-    expected_pair = Enum.sort([{s1, g1}, {s2, g2}])
-
     from(c in Contact,
       where:
         c.band == ^band_decimal and
           c.qso_timestamp >= ^min_ts and
           c.qso_timestamp <= ^max_ts and
-          c.flagged_invalid == false
+          c.flagged_invalid == false and
+          fragment("LEAST(UPPER(station1), UPPER(station2)) = ?", ^min(s1, s2)) and
+          fragment("GREATEST(UPPER(station1), UPPER(station2)) = ?", ^max(s1, s2)) and
+          fragment("LEAST(UPPER(grid1), UPPER(grid2)) = ?", ^min(g1, g2)) and
+          fragment("GREATEST(UPPER(grid1), UPPER(grid2)) = ?", ^max(g1, g2))
     )
-    |> Repo.all()
-    |> Enum.find(fn c ->
-      pair =
-        Enum.sort([
-          {String.upcase(c.station1), String.upcase(c.grid1)},
-          {String.upcase(c.station2), String.upcase(c.grid2)}
-        ])
-
-      pair == expected_pair
-    end)
+    |> Repo.one()
   end
 
   @refinement_fields ~w(grid1 grid2 mode)a
@@ -1101,6 +1094,7 @@ defmodule Microwaveprop.Radio do
   def pending_edit_for_user(contact_id, user_id) do
     ContactEdit
     |> where([e], e.contact_id == ^contact_id and e.user_id == ^user_id and e.status == :pending)
+    |> preload([:user, :contact])
     |> Repo.one()
   end
 

lib/microwaveprop_web/controllers/scores_controller.ex

@@ -103,9 +103,12 @@ defmodule MicrowavepropWeb.ScoresController do
 
     header = <<"PSCR", 1::8, 0::8, 0::8, 0::8, cell_count::little-32>>
 
-    lats = scores |> Enum.map(&<<&1.lat * 1.0::little-float-32>>) |> IO.iodata_to_binary()
-    lons = scores |> Enum.map(&<<&1.lon * 1.0::little-float-32>>) |> IO.iodata_to_binary()
-    score_bytes = scores |> Enum.map(&<<clamp_score(&1.score)::8>>) |> IO.iodata_to_binary()
+    {lats, lons, score_bytes} =
+      Enum.reduce(scores, {[], [], []}, fn s, {lats_acc, lons_acc, scs_acc} ->
+        {[<<s.lat * 1.0::little-float-32>> | lats_acc],
+         [<<s.lon * 1.0::little-float-32>> | lons_acc],
+         [<<clamp_score(s.score)::8>> | scs_acc]}
+      end)
 
     <<header::binary, lats::binary, lons::binary, score_bytes::binary>>
   end