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>

.env.example

@@ -7,10 +7,16 @@ DB_USER=tinkertickets
 DB_PASS=your_password_here
 DB_NAME=ticketing_system
 
-# Discord Webhook (optional - for notifications)
-DISCORD_WEBHOOK_URL=
+# Matrix Webhook (optional - for notifications via matrix-hookshot)
+# Set to your hookshot generic webhook URL, e.g.:
+#   https://matrix.lotusguild.org/webhook/<uuid>
+MATRIX_WEBHOOK_URL=
 
-# Application Domain (required for Discord webhook links)
+# Matrix users to @mention on every new ticket (comma-separated Matrix user IDs)
+# e.g. @jared:matrix.lotusguild.org,@alice:matrix.lotusguild.org
+MATRIX_NOTIFY_USERS=
+
+# Application Domain (required for Matrix webhook ticket links)
 # Set this to your public domain (e.g., t.lotusguild.org)
 APP_DOMAIN=