Add connection verification to prevent false success on MQTT failures

- Add wait_for_connection() to test actual MQTT connectivity after start_link
- Use publish_sync with timeout to verify broker is reachable
- Properly fail startup if MQTT broker is unavailable
- Add clear timeout and connection error messages

This fixes the issue where systant reported 'connected successfully'
even when the MQTT broker was unreachable.

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

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

server/lib/systant/mqtt_client.ex

@@ -36,7 +36,12 @@ defmodule Systant.MqttClient do
 
     case Tortoise.Connection.start_link(connection_opts) do
       {:ok, _pid} ->
-        Logger.info("MQTT client connected successfully")
+        Logger.info("MQTT client process started, verifying connection...")
+        
+        # Wait a bit to verify the connection actually works
+        case wait_for_connection(mqtt_config.client_id, 5000) do
+          :ok ->
+            Logger.info("MQTT connection verified successfully")
             
             # Send system metrics after a short delay to ensure dashboard is ready
             startup_delay = Systant.Config.get(app_config, ["general", "startup_delay"]) || 5000
@@ -50,8 +55,17 @@ defmodule Systant.MqttClient do
             schedule_stats_publish(mqtt_config.publish_interval)
             {:ok, state_config}
             
+          :timeout ->
+            Logger.error("MQTT connection timeout - broker at #{mqtt_config.host}:#{mqtt_config.port} is not responding")
+            {:stop, :connection_timeout}
+            
           {:error, reason} ->
-        Logger.error("Failed to connect to MQTT broker: #{inspect(reason)}")
+            Logger.error("MQTT connection verification failed: #{inspect(reason)}")
+            {:stop, reason}
+        end
+        
+      {:error, reason} ->
+        Logger.error("Failed to start MQTT client: #{inspect(reason)}")
         {:stop, reason}
     end
   end
@@ -131,4 +145,22 @@ defmodule Systant.MqttClient do
   defp schedule_stats_publish(interval) do
     Process.send_after(self(), :publish_stats, interval)
   end
+
+  defp wait_for_connection(client_id, timeout_ms) do
+    # Try to publish a test message to verify the connection
+    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}
+    end
+  end
 end