Fix catch-up checks across partial minutes

[bensynapse]

I run Live Tennis API.

has_been("1 * * * *", since, now) returns false for 12:00:59 through 12:01:01. Its minute steps retain the starting seconds and skip the final minute.

The scan now starts on the minute boundary after validating the original timestamps. Tests cover microseconds, hour and year boundaries, and the documented datetime libraries. Reversed intervals still raise, including when both timestamps fall in one minute.

The four boundary regressions fail on main. uv run coverage run -m unittest discover -b passes all 22 tests on Python 3.10 and 3.14. uv run coverage report --fail-under=100 reports 100% on both versions.

uv run flake8 pycron --max-line-length=88, uv run pylint pycron, uv run mypy pycron and uv build also pass.