Replace Discord webhook notifications with Matrix (hookshot)

- Add helpers/NotificationHelper.php: shared Matrix webhook sender
  that reads MATRIX_WEBHOOK_URL and MATRIX_NOTIFY_USERS from config
- Remove sendDiscordWebhook() from TicketController; call
  NotificationHelper::sendTicketNotification() instead
- Replace 60-line Discord embed block in create_ticket_api.php
  with a single NotificationHelper call
- config/config.php: DISCORD_WEBHOOK_URL → MATRIX_WEBHOOK_URL +
  new MATRIX_NOTIFY_USERS key (comma-separated Matrix user IDs)
- .env.example: updated env var names and comments

Payload sent to hookshot includes notify_users array so the
JS transform can build proper @mention links for each user.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/config.php

@@ -31,8 +31,10 @@ $GLOBALS['config'] = [
     'ASSETS_URL' => '/assets', // Assets URL
     'API_URL' => '/api', // API URL
 
-    // Discord webhook
-    'DISCORD_WEBHOOK_URL' => $envVars['DISCORD_WEBHOOK_URL'] ?? null,
+    // Matrix webhook (hookshot generic webhook URL)
+    'MATRIX_WEBHOOK_URL'   => $envVars['MATRIX_WEBHOOK_URL'] ?? null,
+    // Comma-separated Matrix user IDs to @mention on new tickets (e.g. @jared:matrix.lotusguild.org)
+    'MATRIX_NOTIFY_USERS'  => $envVars['MATRIX_NOTIFY_USERS'] ?? '',
 
     // Domain settings for external integrations (webhooks, links, etc.)
     // Set APP_DOMAIN in .env to override