Clean up legacy singleton references after InvoiceService migration

- Update commented TODO in PaymentMonitorService to use proper DI pattern
- Remove outdated reference to legacy invoiceService singleton
- Prepare PaymentMonitorService for future DI migration with proper pattern

This cleanup eliminates the last remaining reference to the legacy
InvoiceService singleton pattern, ensuring code consistency.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/modules/market/services/paymentMonitor.ts

@@ -168,8 +168,9 @@ class PaymentMonitorService {
         payment_hash: paymentHash
       }
       
-      // Uncomment when wallet context is available:
-      // const status = await invoiceService.checkPaymentStatus(paymentHash, walletId, adminKey)
+      // TODO: Implement when wallet context is available and PaymentMonitorService is migrated to DI:
+      // const invoiceService = injectService(SERVICE_TOKENS.INVOICE_SERVICE) as InvoiceService
+      // const status = await invoiceService.checkPaymentStatus(paymentHash, adminKey)
       // return status
     } catch (error) {
       console.error('Failed to get payment status:', error)