EventPass Update - Midtrans API Key Standar

Update:
- Tambah setting Midtrans API Key Standar:
  Merchant ID, Client Key, Server Key, Environment Sandbox/Production.
- Payment Mode:
  1. Snap API Redirect (rekomendasi)
  2. Payment Link Manual
- Jika memakai Snap API Redirect:
  - peserta klik bayar
  - sistem membuat transaksi Midtrans unik per peserta
  - order_id = invoice_number peserta
  - Midtrans redirect_url otomatis dibuka
  - callback Midtrans bisa auto set paid karena order_id cocok dengan invoice
- Callback endpoint:
  /callback/midtrans.php

Catatan:
- Tidak perlu update database.
- Setting disimpan di uploads/settings/midtrans-api-settings.json.
- Jangan share Server Key ke publik.
- Pastikan PHP cURL aktif di hosting.
- Tanpa install.php dan tanpa config.
