fix: Evitar criação de faturas duplicadas no webhook PayPal

🐛 PROBLEMA IDENTIFICADO:
- Cliente pagava 1 assinatura mas recebia 2 faturas
- Fatura 1: subscription_create (método confirm())
- Fatura 2: subscription_cycle (webhook PAYMENT.SALE.COMPLETED)
- Ambas criadas com 8 segundos de diferença

✅ SOLUÇÃO IMPLEMENTADA:

handlePaymentCompleted() agora verifica:

1. Se já existe fatura com mesmo paypal_payment_id
   - Evita duplicação por webhook reprocessado

2. Se já existe fatura paga HOJE para esta subscription
   - Evita duplicação do pagamento inicial
   - Webhook vem depois do confirm()

3. Só cria nova fatura se for pagamento recorrente genuíno

📊 RESULTADO:
- Pagamento inicial: 1 fatura (subscription_create)
- Renovação mensal/anual: 1 fatura (subscription_cycle)
- Webhooks duplicados: ignorados com log

🗑️ LIMPEZA:
- Removida fatura duplicada WM-2025-000002 do user_id 35

backend/app/Http/Controllers/Api/SubscriptionController.php

@@ -843,6 +843,38 @@ private function handlePaymentCompleted(array $resource): void
         $subscription = Subscription::where('paypal_subscription_id', $billingAgreementId)->first();
         if (!$subscription) return;
 
+        $paymentId = $resource['id'] ?? null;
+
+        // Check if invoice already exists for this payment (avoid duplicates)
+        if ($paymentId) {
+            $existingInvoice = Invoice::where('paypal_payment_id', $paymentId)
+                ->orWhere('paypal_capture_id', $paymentId)
+                ->first();
+
+            if ($existingInvoice) {
+                Log::info('Invoice already exists for this payment', [
+                    'payment_id' => $paymentId,
+                    'invoice_id' => $existingInvoice->id,
+                ]);
+                return;
+            }
+        }
+
+        // Check if a paid invoice was already created for this subscription today (initial payment)
+        $todayInvoice = Invoice::where('subscription_id', $subscription->id)
+            ->where('status', Invoice::STATUS_PAID)
+            ->whereDate('created_at', today())
+            ->first();
+
+        if ($todayInvoice) {
+            Log::info('Invoice already created today for subscription, skipping duplicate', [
+                'subscription_id' => $subscription->id,
+                'existing_invoice_id' => $todayInvoice->id,
+                'payment_id' => $paymentId,
+            ]);
+            return;
+        }
+
         // Create invoice for recurring payment
         $invoice = Invoice::createForSubscription(
             $subscription,