perf: fix remaining medium/low severity inefficiencies

- Radio.group_reciprocals: replace O(n²) nested Enum.filter with O(n)
  Enum.group_by using a hash key (sorted station pair + band + hour)
- contact_live/index.ex: cache monthly bars Repo.all(Contact) query
  with 60s TTL
- ScoreCache.valid_times: replace :ets.select with :ets.foldl to
  avoid intermediate list allocation
- Accounts.list_users: add limit(100) to unbounded query
- Accounts.backfill_missing_home_qth: batch individual Oban.insert
  calls into single Oban.insert_all

lib/microwaveprop/accounts.ex

@@ -90,10 +90,10 @@ defmodule Microwaveprop.Accounts do
 
   ## Admin user management
 
-  @doc "Returns all users ordered by callsign."
+  @doc "Returns users ordered by callsign, limited to 100."
   @spec list_users() :: [User.t()]
   def list_users do
-    Repo.all(from u in User, order_by: [asc: u.callsign])
+    Repo.all(from u in User, order_by: [asc: u.callsign], limit: 100)
   end
 
   @doc "Updates admin-managed user fields (callsign, name, email, is_admin)."
@@ -168,9 +168,8 @@ defmodule Microwaveprop.Accounts do
       |> where([u], is_nil(u.home_grid))
       |> select([u], u.id)
       |> Repo.all()
-      |> Enum.each(fn id ->
-        _ = %{user_id: id} |> worker.new() |> Oban.insert()
-      end)
+      |> Enum.map(fn id -> %{user_id: id} |> worker.new() end)
+      |> then(&if &1 != [], do: Oban.insert_all(&1))
     end
 
     :ok

lib/microwaveprop/propagation/score_cache.ex

@@ -149,10 +149,15 @@ 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
-    match_spec = [{{{band_mhz, :"$1"}, :_}, [], [:"$1"]}]
-
     @table
-    |> :ets.select(match_spec)
+    |> :ets.foldl(
+      fn
+        {{^band_mhz, vt}, _}, acc -> [vt | acc]
+        _, acc -> acc
+      end,
+      [],
+      @table
+    )
     |> Enum.sort({:asc, DateTime})
   end
 

lib/microwaveprop/radio.ex

@@ -230,42 +230,20 @@ defmodule Microwaveprop.Radio do
   """
   @spec group_reciprocals([Contact.t()]) :: [{Contact.t(), [Contact.t()]}]
   def group_reciprocals(contacts) do
-    {groups, _seen_ids} =
-      Enum.reduce(contacts, {[], MapSet.new()}, fn contact, {groups, seen} ->
-        group_contact(contact, contacts, groups, seen)
-      end)
-
-    Enum.reverse(groups)
+    contacts
+    |> Enum.group_by(&reciprocal_key/1)
+    |> Enum.map(fn {_key, [primary | rest]} -> {primary, rest} end)
   end
 
-  defp group_contact(contact, contacts, groups, seen) do
-    if MapSet.member?(seen, contact.id) do
-      {groups, seen}
-    else
-      reciprocals =
-        Enum.filter(contacts, fn other ->
-          other.id != contact.id and
-            not MapSet.member?(seen, other.id) and
-            other.band == contact.band and
-            reciprocal_pair?(contact, other)
-        end)
-
-      new_seen = Enum.reduce(reciprocals, MapSet.put(seen, contact.id), &MapSet.put(&2, &1.id))
-      {[{contact, reciprocals} | groups], new_seen}
-    end
+  defp reciprocal_key(contact) do
+    stations = Enum.sort([contact.station1, contact.station2])
+    {stations, contact.band, reciprocal_hour_key(contact.qso_timestamp)}
   end
 
-  defp reciprocal_pair?(a, b) do
-    same_stations =
-      (a.station1 == b.station2 and a.station2 == b.station1) or
-        (a.station1 == b.station1 and a.station2 == b.station2)
+  defp reciprocal_hour_key(nil), do: nil
 
-    same_stations and same_hour?(a.qso_timestamp, b.qso_timestamp)
-  end
-
-  defp same_hour?(t1, t2) do
-    t1 |> NaiveDateTime.truncate(:second) |> Map.take([:year, :month, :day, :hour]) ==
-      t2 |> NaiveDateTime.truncate(:second) |> Map.take([:year, :month, :day, :hour])
+  defp reciprocal_hour_key(%_{} = ts) do
+    Map.take(ts, [:year, :month, :day, :hour])
   end
 
   # Only cache the unscoped, unsearched total — scope-dependent counts

lib/microwaveprop_web/live/contact_live/index.ex

@@ -7,6 +7,7 @@ defmodule MicrowavepropWeb.ContactLive.Index do
 
   alias Microwaveprop.Accounts.Scope
   alias Microwaveprop.Accounts.User
+  alias Microwaveprop.Cache
   alias Microwaveprop.Radio.Contact
   alias Microwaveprop.Repo
 
@@ -77,11 +78,15 @@ defmodule MicrowavepropWeb.ContactLive.Index do
      |> assign(:data_provider, {__MODULE__, :visible_query_provider, [scope_token(scope)]})}
   end
 
+  @monthly_bars_cache_key {__MODULE__, :monthly_bars}
+  @monthly_bars_ttl_ms 60_000
+
   # Counts every contact in the corpus regardless of viewer scope —
   # the chart is a public-facing seasonal summary, not a personal
   # view, so private contacts roll into the per-month totals like
-  # any other.
+  # any other. Cached for 60s since data only changes on new inserts.
   defp monthly_bars do
+    Cache.fetch_or_store(@monthly_bars_cache_key, @monthly_bars_ttl_ms, fn ->
       by_month =
         Contact
         |> where([c], not is_nil(c.qso_timestamp))
@@ -106,6 +111,7 @@ defmodule MicrowavepropWeb.ContactLive.Index do
         height: height
       }
     end)
+    end)
   end
 
   defp bar_height(_count, 0), do: 0