Fix predicate function naming conventions

Rename predicate functions to follow Elixir style guide:
- Remove 'is_' prefix from predicate function names
- 'is_' prefix is reserved for guard-safe functions

Changes:
- ip_geolocation.ex: is_private_ip? → private_ip?
- ip_geolocation.ex: is_172_private_range? → in_172_private_range?
- leader_election.ex: is_pid_alive? → pid_alive?

Fixes 3 Credo code readability issues.
All Credo code readability issues now resolved.

lib/aprsme/cluster/leader_election.ex

@@ -205,7 +205,7 @@ defmodule Aprsme.Cluster.LeaderElection do
   end
 
   defp check_pid_liveness(pid, pid_node) do
-    if is_pid_alive?(pid, pid_node) do
+    if pid_alive?(pid, pid_node) do
       :ok
     else
       reason = if pid_node == node(), do: "dead local process", else: "dead remote process"
@@ -216,11 +216,11 @@ defmodule Aprsme.Cluster.LeaderElection do
       cleanup_registration("problematic process #{inspect(pid)}")
   end
 
-  defp is_pid_alive?(pid, pid_node) when pid_node == node() do
+  defp pid_alive?(pid, pid_node) when pid_node == node() do
     Process.alive?(pid)
   end
 
-  defp is_pid_alive?(pid, pid_node) do
+  defp pid_alive?(pid, pid_node) do
     if :rpc.call(pid_node, Process, :alive?, [pid]) do
       true
     else

lib/aprsme_web/plugs/ip_geolocation.ex

@@ -128,24 +128,24 @@ defmodule AprsmeWeb.Plugs.IPGeolocation do
   end
 
   defp valid_ip_for_geolocation?(ip) when is_binary(ip) do
-    not is_private_ip?(ip)
+    not private_ip?(ip)
   end
 
   defp valid_ip_for_geolocation?(_), do: false
 
-  defp is_private_ip?(ip) do
+  defp private_ip?(ip) do
     cond do
       String.starts_with?(ip, "127.") -> true
       String.starts_with?(ip, "::1") -> true
       String.starts_with?(ip, "10.") -> true
       String.starts_with?(ip, "192.168.") -> true
-      String.starts_with?(ip, "172.") -> is_172_private_range?(ip)
+      String.starts_with?(ip, "172.") -> in_172_private_range?(ip)
       true -> false
     end
   end
 
   # Check if IP is in 172.16.0.0/12 range (172.16.0.0 - 172.31.255.255)
-  defp is_172_private_range?(ip) do
+  defp in_172_private_range?(ip) do
     case String.split(ip, ".") do
       ["172", second_octet | _] ->
         case Integer.parse(second_octet) do