Implement graceful shutdown for MQTT connection failures

- Added connection verification with timeout-based validation on startup
- Enhanced error handling to catch Tortoise.publish_sync exceptions
- Graceful exit via System.stop(1) prevents erl_crash.dump files
- Added comprehensive logging for connection failures and shutdown reasons
- Updated CLAUDE.md to document graceful shutdown behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

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

server/lib/systant/mqtt_client.ex

@@ -57,10 +57,14 @@ defmodule Systant.MqttClient do
             
           :timeout ->
             Logger.error("MQTT connection timeout - broker at #{mqtt_config.host}:#{mqtt_config.port} is not responding")
+            Logger.error("Shutting down systant due to MQTT connection failure")
+            System.stop(1)
             {:stop, :connection_timeout}
             
           {:error, reason} ->
             Logger.error("MQTT connection verification failed: #{inspect(reason)}")
+            Logger.error("Shutting down systant due to MQTT connection failure")
+            System.stop(1)
             {:stop, reason}
         end
         
@@ -151,16 +155,29 @@ defmodule Systant.MqttClient do
     test_topic = "systant/connection_test"
     test_payload = "test"
     
-    case Tortoise.publish_sync(client_id, test_topic, test_payload, qos: 0, timeout: timeout_ms) do
-      :ok ->
-        Logger.debug("MQTT connection test successful")
-        :ok
-      {:error, :timeout} ->
-        Logger.error("MQTT connection test timed out")
-        :timeout
-      {:error, reason} ->
-        Logger.error("MQTT connection test failed: #{inspect(reason)}")
-        {:error, reason}
+    try do
+      case Tortoise.publish_sync(client_id, test_topic, test_payload, qos: 0, timeout: timeout_ms) do
+        :ok ->
+          Logger.debug("MQTT connection test successful")
+          :ok
+        {:error, :timeout} ->
+          Logger.error("MQTT connection test timed out")
+          :timeout
+        {:error, reason} ->
+          Logger.error("MQTT connection test failed: #{inspect(reason)}")
+          {:error, reason}
+        other ->
+          Logger.error("MQTT connection test unexpected result: #{inspect(other)}")
+          {:error, other}
+      end
+    rescue
+      error ->
+        Logger.error("MQTT connection test exception: #{inspect(error)}")
+        {:error, :connection_failed}
+    catch
+      :exit, reason ->
+        Logger.error("MQTT connection test exit: #{inspect(reason)}")
+        {:error, :connection_failed}
     end
   end
 end