Fix ShutdownHandler terminating during normal startup

- Only initiate graceful shutdown for abnormal termination reasons
- Ignore :shutdown, {:shutdown, _}, and :normal termination reasons
- Add logging for SIGTERM handler installation

Co-Authored-By: Claude <noreply@anthropic.com>

lib/aprsme/shutdown_handler.ex

@@ -69,7 +69,20 @@ defmodule Aprsme.ShutdownHandler do
   def terminate(reason, state) do
     Logger.info("ShutdownHandler terminating: #{inspect(reason)}")
 
-    if not state.shutting_down do
+    # Only initiate shutdown for specific reasons, not during normal shutdown
+    should_graceful_shutdown =
+      case reason do
+        # Normal shutdown, don't interfere
+        :shutdown -> false
+        # Normal shutdown with reason
+        {:shutdown, _} -> false
+        # Normal termination
+        :normal -> false
+        # Abnormal termination, do graceful shutdown
+        _ -> true
+      end
+
+    if should_graceful_shutdown and not state.shutting_down do
       initiate_shutdown(state)
       # Give some time for the shutdown process
       Process.sleep(state.drain_timeout)

lib/aprsme/signal_handler.ex

@@ -13,7 +13,13 @@ defmodule Aprsme.SignalHandler do
 
   def init(_opts) do
     # Install signal handler for SIGTERM
-    :ok = :os.set_signal(:sigterm, :handle)
+    case :os.set_signal(:sigterm, :handle) do
+      :ok ->
+        Logger.info("SIGTERM signal handler installed")
+
+      error ->
+        Logger.warning("Failed to install SIGTERM handler: #{inspect(error)}")
+    end
 
     {:ok, %{}}
   end