Add PEP 561 marker (py.typed)

[MatthewCane]

Summary

Add an empty python_ntfy/py.typed file to mark the package as typed per PEP 561. This enables downstream projects to consume our inline type hints by default.

Details

• The file is intentionally empty; its presence signals to type checkers (mypy, Pyright) that the package is typed.
• Verified with just build that the wheel contains python_ntfy/py.typed.

Issue

Closes #164

Test plan

• just check and just mypy should pass
• just build then inspect the wheel to confirm python_ntfy/py.typed is present