fix bugs in backfill start/end TZ handling (#653)

Two bugs:
- When end time has no TZ, it'll throw when compared to start
- When start time has no tz, `astimezone(tz)` first converts it to the
machine's local time, not UTC (then it is converted to `tz`)

<img width="659" height="353" alt="Screenshot 2026-04-27 at 13 18 00"
src="https://github.com/user-attachments/assets/62d3c6b5-0129-4da7-8caa-25b6fc75a924"
/>