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/rollout/src/recorder.rs

@@ -1252,7 +1252,7 @@ impl From<codex_state::ThreadsPage> for ThreadsPage {
                 model_provider: Some(item.model_provider),
                 cli_version: Some(item.cli_version),
                 created_at: Some(item.created_at.to_rfc3339_opts(SecondsFormat::Secs, true)),
-                updated_at: Some(item.updated_at.to_rfc3339_opts(SecondsFormat::Secs, true)),
+                updated_at: Some(item.updated_at.to_rfc3339_opts(SecondsFormat::Millis, true)),
             })
             .collect();
         Self {