Bugs fixed:
- Step 2 was skipped: checkDuplicates() advanced step AND nextStep() advanced
  again after it returned. Now nextStep() returns early after checkDuplicates()
- No invoice was ever created: confirm() now creates a real invoice via
  InvoiceService with the price resolved from base_prices table
- No payment was ever recorded: now uses PaymentService.recordPayment() which
  creates double-entry transaction and updates invoice paid_amount
- fee_amount doesn't exist on training_programs — price comes from base_prices
  (polymorphic). Added resolveProgramFee() + selectedProgramFee computed property
- Success screen now shows: participant number, program, invoice number/amount,
  payment status (paid vs outstanding)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>