Support timezone offsets in datetime fields

Timestamps with timezone offsets like +02:00 or +05:30 now work across all API endpoints. You no longer need to convert to UTC before uploading.

• Accepted timezone offsets on all datetime input fields: run timestamps, phase and log timestamps, and date filters on runs, units, batches, and procedures
• Responses still return UTC (Z suffix), no changes needed on the read side

from datetime import datetime, timezone, timedelta

# UTC (always worked)
client.runs.create(
    started_at=datetime.now(timezone.utc),
    ended_at=datetime.now(timezone.utc),
    ...
)

# Local timezone (now works too)
cet = timezone(timedelta(hours=2))
client.runs.create(
    started_at=datetime.now(cet),
    ended_at=datetime.now(cet),
    ...
)