The StripeInvoiceService was only creating line items for tickets but missing
the 1€ service fee, causing a discrepancy where customers paid 26€ via Stripe
checkout but the generated invoice only showed 25€.
- Add service fee line item to Stripe invoices in StripeInvoiceService
- Update all related tests to expect two line items (tickets + service fee)
- Fix order controller test to account for service fee in total calculation
Now Stripe invoices properly match the amount paid: tickets + 1€ service fee.
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
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