Revenue model: 3% fee on every POS transaction, shown as "مصاريف خدمة"
on receipts and invoices. The percentage is controlled ONLY via the
PLATFORM_SERVICE_FEE_PERCENT env var — not editable by any admin.

- PlatformFeeService: calculates fee from env var (default 3%)
- POSService: includes service_fee_amount in transaction + invoice total
- POS terminal UI: shows fee line before grand total
- Receipt print: displays "مصاريف خدمة" row
- Invoice show: displays service fee row
- System settings: read-only banner showing the current fee percentage
- Migration: adds service_fee_amount column to pos_transactions + invoices
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>