Implement Discord and Slack notifications for call events

Settings & Configuration:
- Added Discord webhook URL, Slack webhook URL settings in admin
- Added notification type toggles (incoming calls, queue timeouts, missed calls)
- Added queue timeout threshold setting (30-1800 seconds)
- Registered all new settings with WordPress options system

Notification System:
- Created TWP_Notifications class for Discord/Slack webhook handling
- Rich message formatting with embeds/attachments for both platforms
- Color-coded notifications (blue=incoming, yellow=timeout, red=missed)
- Comprehensive error handling and logging

Integration Points:
- Incoming calls: Notifications sent when calls enter queues
- Queue timeouts: Automated monitoring via cron job (every minute)
- Missed calls: Notifications for browser phone and general missed calls
- Added notified_timeout column to prevent duplicate timeout notifications

Features:
- Professional Discord embeds with fields and timestamps
- Slack attachments with proper formatting and colors
- Automatic cron job setup for queue timeout monitoring
- Fallback to SMS notifications while Discord/Slack also work
- Configurable notification types and timeout thresholds

This provides real-time call notifications to Discord channels and Slack channels,
helping teams stay informed about incoming calls and queue issues even when
SMS notifications aren't working due to validation delays.

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

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

includes/class-twp-call-queue.php

@@ -574,6 +574,15 @@ class TWP_Call_Queue {
             return;
         }
         
+        // Send Discord/Slack notification for incoming call
+        require_once dirname(__FILE__) . '/class-twp-notifications.php';
+        TWP_Notifications::send_call_notification('incoming_call', array(
+            'type' => 'incoming_call',
+            'caller' => $caller_number,
+            'queue' => $queue->queue_name,
+            'queue_id' => $queue_id
+        ));
+        
         // Get members of the assigned agent group
         require_once dirname(__FILE__) . '/class-twp-agent-groups.php';
         $members = TWP_Agent_Groups::get_group_members($queue->agent_group_id);