Modernize type annotations: replace Optional with union syntax

[Copilot]

Addresses feedback on PR #40 to use Python 3.10+ union syntax for type annotations instead of typing.Optional.

Changes

• Replace Optional[InvoiceRequest] with InvoiceRequest | None in app/routers/operator.py
• Remove unused Optional import from typing module

# Before
from typing import Optional

async def invoice(
    data: Optional[InvoiceRequest] = None,
    ...
) -> InvoiceResponse:

# After
async def invoice(
    data: InvoiceRequest | None = None,
    ...
) -> InvoiceResponse:

Aligns with project's Python 3.11+ requirement and modern type annotation standards.

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[Neiland85]

Closing this PR. It is now outdated after the full system refactor completed in December 2025.

The project has been unified under a new architecture, updated dependencies, and a consistent configuration model.

Keeping this PR open would introduce inconsistencies and no longer provides technical value.