Add AST JSON representation for SiteConfigController.php with nodes and edges

docs/api/admin.md

@@ -847,9 +847,11 @@ Returns all site configuration values.
     "appointment_reminder_hours": "2",
     "log_retention_days": "90",
     "payment_test_mode": "0",
+    "mellat_enabled": "1",
     "mellat_terminal_id": "",
     "mellat_username": "",
     "mellat_password": "",
+    "sep_enabled": "1",
     "sep_terminal_id": "",
     "sms_provider": "kavenegar",
     "kavenegar_api_key": "",
@@ -892,6 +894,7 @@ Update one or more settings. Unknown keys are silently ignored.
 
 **Payment gateway rules:**
 - `payment_test_mode` — `"1"` = all payments use MockGateway (no real bank calls), `"0"` = real gateways
+- `mellat_enabled` / `sep_enabled` — `"1"` = gateway selectable, `"0"` = gateway disabled (hidden from the payment selection list and rejected at `initiate`). Unset = enabled (default).
 - `payment_allowed_frontend_hosts` — comma-separated hosts allowed as a payment `frontend_address` (origin site to return to). Falls back to the `ALLOWED_FRONTEND_HOSTS` env var when empty. Add a consumer site's host here to permit its payments.
 - Gateway credentials (mellat/sep) read from DB first, fallback to env vars if DB value is empty
 - MockGateway callback: same URL pattern + `&mock=1&ResCode=0&RefId=MOCK-{orderId}` (add `&cancel=1` to simulate a user cancellation → `canceled`)