SQLite/aiosqlite returns DateTime columns as naive even though every
value is written in UTC, so a bare .isoformat() dropped the offset and
the frontend's new Date() parsed it as local time. Add a shared
isoformat_utc() helper and use it at every call site that was missing
the fix already applied ad hoc in rounds.py.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
