feat(payment): implement OxaPay provider and non-recurring subscription sync

- Add comprehensive OxaPay payment provider with invoice creation, webhook processing, and subscription status sync
  - Implement conditional payload fields (to_currency, callback_url) based on configuration
  - Create universal sync command for all non-recurring payment providers
  - Add subscription model fields for payment tracking
  - Implement proper status mapping between OxaPay and Laravel subscription states
  - Add webhook signature validation using HMAC SHA512

app/Models/Subscription.php

@@ -62,6 +62,9 @@ class Subscription extends Model
         'customer_metadata',
         'trial_will_end_sent_at',
         'pause_reason',
+        'amount',
+        'currency',
+        'expires_at',
     ];
 
     protected $casts = [