Add missing processedBy relationship to Payment model

Used by ReceiptService for cashier name on payment receipts.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Domain/Financial/Models/Payment.php

@@ -90,6 +90,11 @@ public function cashSession(): BelongsTo
         return $this->belongsTo(CashSession::class);
     }
 
+    public function processedBy(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'received_by');
+    }
+
     public function isInbound(): bool
     {
         return $this->direction === 'inbound';