## Why
New non-ephemeral threads have no persisted rollout or preview until their first
turn, so moving them into a section could leave them absent from section-filtered
thread lists.
## What changed
- Materialize and flush a non-ephemeral thread before applying an explicit
section move.
- Include explicitly sectioned threads with empty previews in section-filtered
queries, and update the partial indexes to cover them.
## Testing
- Add an app-server test that moves a newly started thread into and out of the
pinned section before its first turn.
- Cover empty-preview threads in recency and section-position query tests.
GitOrigin-RevId: aa3c4daf9890f06c08b93cd60f146099cbc6cfb2
