Add Mattermost notification channel support

Introduces Mattermost as a new notification channel, including backend service integration, controller validation, UI form fields, and updates to channel type enums in the database schema and migrations. This enables users to configure and send notifications via Mattermost webhooks.

app/Services/Channels/MattermostChannel.php

@@ -0,0 +1,107 @@
+<?php
+
+namespace App\Services\Channels;
+
+use GuzzleHttp\Client;
+use App\Services\Logger;
+
+class MattermostChannel implements NotificationChannelInterface
+{
+    private Client $client;
+    private Logger $logger;
+
+    public function __construct()
+    {
+        $this->client = new Client(['timeout' => 10]);
+        $this->logger = new Logger('mattermost_channel');
+    }
+
+    public function send(array $config, string $message, array $data = []): bool
+    {
+        if (!isset($config['webhook_url'])) {
+            return false;
+        }
+
+        try {
+            // Mattermost expects a simple text payload or attachments
+            $payload = [
+                'text' => $message
+            ];
+
+            // Add attachments for richer formatting if domain data is available
+            if (isset($data['domain'])) {
+                $color = $this->getColorByDaysLeft($data['days_left'] ?? null);
+                
+                $payload['attachments'] = [
+                    [
+                        'color' => $color,
+                        'title' => '🔔 Domain Expiration Alert',
+                        'text' => $message,
+                        'fields' => [
+                            [
+                                'short' => true,
+                                'title' => 'Domain',
+                                'value' => $data['domain']
+                            ],
+                            [
+                                'short' => true,
+                                'title' => 'Days Left',
+                                'value' => $data['days_left'] ?? 'N/A'
+                            ],
+                            [
+                                'short' => true,
+                                'title' => 'Expiration Date',
+                                'value' => $data['expiration_date'] ?? 'N/A'
+                            ],
+                            [
+                                'short' => true,
+                                'title' => 'Registrar',
+                                'value' => $data['registrar'] ?? 'N/A'
+                            ]
+                        ],
+                        'footer' => 'Domain Monitor',
+                        'ts' => time()
+                    ]
+                ];
+            }
+
+            $response = $this->client->post($config['webhook_url'], [
+                'json' => $payload
+            ]);
+
+            $ok = $response->getStatusCode() === 200;
+            if ($ok) {
+                $this->logger->info('Mattermost message sent', [
+                    'status' => $response->getStatusCode()
+                ]);
+            } else {
+                $this->logger->error('Mattermost non-200 status', [
+                    'status' => $response->getStatusCode()
+                ]);
+            }
+            return $ok;
+        } catch (\Exception $e) {
+            $this->logger->error('Mattermost send failed', [
+                'exception' => $e->getMessage()
+            ]);
+            return false;
+        }
+    }
+
+    private function getColorByDaysLeft(?int $daysLeft): string
+    {
+        if ($daysLeft === null) {
+            return '#36a64f'; // Green
+        }
+
+        if ($daysLeft <= 0) {
+            return '#ff0000'; // Red - expired
+        } elseif ($daysLeft <= 1) {
+            return '#ff6600'; // Orange - critical
+        } elseif ($daysLeft <= 7) {
+            return '#ffaa00'; // Yellow - warning
+        } else {
+            return '#36a64f'; // Green - ok
+        }
+    }
+}