feat: Migrate alert system from WhatsApp to Slack integration with updated configuration and sender classes

templates/config.html

@@ -47,12 +47,15 @@
       </div>
     </div>
 
-    <h3 class="mt-2">WhatsApp</h3>
+    <h3 class="mt-2">Slack</h3>
     <div class="row">
       <div class="col">
-        <label>Send To (WhatsApp E.164, no spaces)<br>
-          <input type="text" name="whatsapp_to" placeholder="+15551234567" value="{{ cfg.whatsapp_to or '' }}" />
+        <label>Webhook URL<br>
+          <input type="url" name="slack_webhook_url" placeholder="https://hooks.slack.com/services/..." value="{{ cfg.slack_webhook_url or '' }}" />
         </label>
+        <p class="muted" style="font-size: 0.9em; margin-top: 0.25em;">
+          Create a webhook at <a href="https://api.slack.com/apps" target="_blank">api.slack.com/apps</a>
+        </p>
       </div>
     </div>