feat(invoice): create invoices after payment completion

[LeoMoshko]

Automatic e-invoice creation after payment completion

This change implements task 03: automatic e-invoice creation for paid orders.

What was added

• A Symfony event subscriber listening for the Sylius payment complete workflow transition.

• Automatic creation and processing of an e-invoice after successful payment.

• Per-channel configuration using enabled_channels.

• Idempotency protection:

  • repository check before creating an invoice;
  • unique database constraint ensuring one e-invoice per order.

• Doctrine migration for the unique order constraint.

• PHPUnit coverage for enabled channels, disabled channels, duplicate events, and event registration.

• Behat coverage using the real Sylius payment workflow.

• Updated README, changelog, architecture documentation, and task status.

Configuration

Automatic creation is disabled by default. Enable it for selected channel codes:

madcoders_sylius_einvoicing:
enabled_channels:
- WEB_US
- WEB_PL

Verification

• PHPStan: passed
• ECS: passed
• PHPUnit: 28 tests, 82 assertions
• Behat: 6 scenarios, 86 steps
• Symfony container lint: passed
• Doctrine mapping validation: passed