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)

resources/views/site/billing.blade.php

@@ -140,6 +140,22 @@
                             </form>
                         @endif
 
+                        @if ($methods['paynet'])
+                            <form method="POST" action="{{ route('pay.start', ['subscription' => $inv->id]) }}" class="method-form">
+                                @csrf
+                                <input type="hidden" name="method" value="paynet">
+                                <button class="method-btn">
+                                    <div class="method">
+                                        <div class="icon">🇲🇩</div>
+                                        <div>
+                                            <div class="label">Paynet</div>
+                                            <div class="desc">Card MAIB / MICB / Victoriabank, MD Cash, e-money</div>
+                                        </div>
+                                    </div>
+                                </button>
+                            </form>
+                        @endif
+
                         @if ($methods['bank'])
                             <form method="POST" action="{{ route('pay.start', ['subscription' => $inv->id]) }}" class="method-form">
                                 @csrf
@@ -156,7 +172,7 @@
                             </form>
                         @endif
 
-                        @if (! $methods['stripe'] && ! $methods['paypal'] && ! $methods['bank'])
+                        @if (! $methods['stripe'] && ! $methods['paypal'] && ! $methods['paynet'] && ! $methods['bank'])
                             <div style="text-align:center;color:#9ca3af;padding:32px 16px;font-size:13px;">
                                 Nicio metodă de plată configurată. Contactează operatorul.
                             </div>