Avoid full metadata rewrites for thread timestamp updates - #48983

## Why

Updates that only change `updated_at` should not rewrite unrelated thread metadata or its indexes.

## What changed

Use `touch_thread_updated_at` for timestamp-only patches when the SQLite row exists and the rollout path is unchanged. Preserve the full update path for missing rows, changed rollout paths, and patches containing other metadata, including staged changes.

## Testing

Add regression coverage for legacy and paginated histories that verifies missing-row reconstruction, timestamp-only writes using SQLite triggers, preservation of other metadata, and application and removal of staged metadata.

GitOrigin-RevId: d5bc70d57e591e3eb8e9521f0c31a7ef75f244d3