chore: import datetime as dt for ruff 0.17

[lesnik512]

Summary

ruff 0.17.0, released 2026-10-09, enforces the datetime to dt import convention by default (ICN001), so ruff check fails on main. This applies ruff's own fix in the 3 file(s) that import datetime: import datetime as dt. None of them already used the name dt.

Test plan

• ruff check and ruff format --check pass with ruff 0.17.0 and with the version the repo locks
• ty
• the test suite passes locally

[lesnik512]

This was generated by AI.

Closing: #18 already imports datetime as dt in all three files, so main passes ruff 0.17.0 without this.