Sync timestamp should increase to microsecond precision

[marcus-crane]

Something I noticed while reviewing my own highlights in Obsidian is that timestamps for highlights from Readwise Reader contain 6 decimal places of precision (microseconds) whereas we just submit regular second timestamps with October.

Functionally, this makes no difference but visually makes for nice consistency and technically it's possible to have ordering weirdness if different books were uploaded within the same second which is probably rate but not impossible.