Add Paynet (Moldova) payment gateway

PaymentSettings:
- New "🇲🇩 Paynet" section: enabled toggle, mode (test/live), merchant_code,
  service_id, user, password, secret (HMAC), webhook URL hint
- Webhook URL: https://service.mir.md/payments/paynet/webhook

PaymentController:
- startPaynet() — builds Paynet redirect (stub mode prints flow)
- paynetWebhook() — verifies HMAC-SHA256 signature canonical
  Merchant_Code|Order_ID|Amount|Status, marks subscription paid on Status=OK,
  matches by invoice_number = Order_ID
- availableMethods() includes paynet

Tenant /billing:
- 4th payment button "🇲🇩 Paynet" — visible only when configured.
  Description: Card MAIB / MICB / Victoriabank, MD Cash, e-money

Routes:
- POST /payments/paynet/webhook (CSRF excluded)

routes/web.php

@@ -43,6 +43,8 @@ Route::post('/payments/stripe/webhook', [\App\Http\Controllers\PaymentController
     ->withoutMiddleware([\Illuminate\Foundation\Http\Middleware\VerifyCsrfToken::class]);
 Route::post('/payments/paypal/webhook', [\App\Http\Controllers\PaymentController::class, 'paypalWebhook'])
     ->withoutMiddleware([\Illuminate\Foundation\Http\Middleware\VerifyCsrfToken::class]);
+Route::post('/payments/paynet/webhook', [\App\Http\Controllers\PaymentController::class, 'paynetWebhook'])
+    ->withoutMiddleware([\Illuminate\Foundation\Http\Middleware\VerifyCsrfToken::class]);
 
 // Stub `login` route — needed because Laravel's auth middleware tries to
 // route('login') when redirecting unauthenticated requests. We don't have a