refactor: leadership_change handler pattern-matches instead of cond

Cluster.ConnectionManager.handle_info({:leadership_change, ...}) used
a cond on the (became_leader, node, connection_started) triple. Split
into handle_leadership_change/3 with three clauses:

- became_leader=true, us=true, not already started → start APRS-IS
- became_leader=false, us=true, already started → stop APRS-IS
- everything else → no-op

Makes the three state transitions visible in the function heads and
eliminates one more cond from the codebase.

lib/aprsme/cluster/connection_manager.ex

@@ -40,25 +40,28 @@ defmodule Aprsme.Cluster.ConnectionManager do
 
   @impl true
   def handle_info({:leadership_change, node, became_leader}, state) do
-    cond do
-      became_leader and node == node() and not state.connection_started ->
-        Logger.info("This node became the leader, starting APRS-IS connection")
-        start_aprs_connection()
-        {:noreply, %{state | connection_started: true}}
-
-      not became_leader and node == node() and state.connection_started ->
-        Logger.info("This node lost leadership, stopping APRS-IS connection")
-        stop_aprs_connection()
-        {:noreply, %{state | connection_started: false}}
-
-      true ->
-        {:noreply, state}
-    end
+    handle_leadership_change(became_leader, node == node(), state)
   end
 
-  @impl true
   def handle_info(_msg, state), do: {:noreply, state}
 
+  # Became the cluster leader and we weren't already running — start APRS-IS.
+  defp handle_leadership_change(true, true, %{connection_started: false} = state) do
+    Logger.info("This node became the leader, starting APRS-IS connection")
+    start_aprs_connection()
+    {:noreply, %{state | connection_started: true}}
+  end
+
+  # Lost leadership while we were running — stop APRS-IS.
+  defp handle_leadership_change(false, true, %{connection_started: true} = state) do
+    Logger.info("This node lost leadership, stopping APRS-IS connection")
+    stop_aprs_connection()
+    {:noreply, %{state | connection_started: false}}
+  end
+
+  # Any other combination (different node, already in the right state) is a no-op.
+  defp handle_leadership_change(_became_leader, _us?, state), do: {:noreply, state}
+
   @impl true
   def terminate(_reason, _state) do
     Phoenix.PubSub.unsubscribe(Aprsme.PubSub, "cluster:leadership")