## What changed
- Replace the `isPinned` thread metadata and filters with an optional persisted
`section` and `sectionId`.
- Add the paginated `threadSection/list` app-server method so clients can
discover sections even when they contain no threads.
- Seed a stable `Pinned` section, validate section assignments, and support
filtering for a specific section or for unsectioned threads.
## Testing
- Cover section protocol serialization, listing and pagination, metadata
updates, filtering, persistence, migration compatibility, and operation
without SQLite state.
GitOrigin-RevId: 7972b5471d29317b9387bfd90aa9f573f691ad4c
