serverdb: Fix false commit conflict on datetimes

[kofrezo]

_validate_commit compared the current value via json_encode_extra()
against the old value via str(). For datetime attributes these never
match, since str() keeps microseconds and the "+00:00" tz form while
json_encode_extra() drops both.

[kofrezo]

This should fix bugs such as:

serveradmin.serverdb.query_committer.CommitNewerData: ["Newer data available for attribute [(9, 'last_updated', datetime.datetime(2026, 7, 1, 13, 25, 31, 987683, tzinfo=datetime.timezone.utc))]"]

[seqizz]

As far as I understand this will solve most (maybe all) errors we're getting for backup backend at least, since we pass datetime object, now being normalized. If I still keep seeing issues, I can dig a bit more, maybe I am doing something extra wrong (while passing both backup_state and backup_datetime attrs in same commit).