Moving updated-at timestamps to unique millisecond times (#17489)

To allow the ability to have guaranteed-unique cursors, we make two
important updates:
* Add new updated_at_ms and created_at_ms columns that are in
millisecond precision
* Guarantee uniqueness -- if multiple items are inserted at the same
millisecond, bump the new one by one millisecond until it becomes unique

This lets us use single-number cursors for forwards and backwards paging
through resultsets and guarantee that the cursor is a fixed point to do
(timestamp > cursor) and get new items only.

This updated implementation is backwards-compatible since multiple
appservers can be running and won't handle the previous method well.

codex-rs/state/src/paths.rs

@@ -1,10 +1,9 @@
 use chrono::DateTime;
-use chrono::Timelike;
 use chrono::Utc;
 use std::path::Path;
 
 pub(crate) async fn file_modified_time_utc(path: &Path) -> Option<DateTime<Utc>> {
     let modified = tokio::fs::metadata(path).await.ok()?.modified().ok()?;
     let updated_at: DateTime<Utc> = modified.into();
-    Some(updated_at.with_nanosecond(0).unwrap_or(updated_at))
+    Some(updated_at)
 }