feat(6849): the command-line restore uses the same rollback, and the docs say how restore works now

python -m grimoire.restore <file> prints what the restore would change (the
same preview as Settings) and stops. With --yes it runs
backup_job.rollback_to_backup: the current state is saved first, then the
whole instance goes back to the backup. An empty instance takes any full
backup. A live one only takes a backup of its own, and otherwise exits 3 with
the reason. A v1 backup keeps the copy-in path into an empty instance.

docs/backups.md Restoring is rewritten:
- what a restore puts back and removes;
- the same-instance rule;
- the safety backup;
- what survives;
- the web and command-line ways in.

Its S3 key advice now asks for read as well as write when restoring from the
bucket. README, features, architecture and the API reference follow.

Milestone 582, step 5.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/features.md

@@ -85,8 +85,8 @@ Share a project, note or task with users or groups, as viewer, editor or admin.
 ## Export and backup
 
 - **Personal export**: your notes and tasks as a Markdown ZIP (with YAML frontmatter) or JSON.
-- **Admin backup and restore**, with ids remapped so a backup restores onto another instance.
-- **Scheduled backups**: a daily, dated, uncompressed tar to a folder or an S3-compatible bucket, laid out so a deduplicating tool such as restic stores only what changed. `python -m grimoire.restore` brings a fresh instance back from one. See [Backups](backups.md).
+- **Admin backup and restore**: a restore rolls the whole instance back to one of its own backups, after saving the current state, or brings a new instance back from any backup.
+- **Scheduled backups**: a daily, dated, uncompressed tar to a folder or an S3-compatible bucket, laid out so a deduplicating tool such as restic stores only what changed. Restoring is in Settings, or `python -m grimoire.restore`. See [Backups](backups.md).
 
 ## Sign-in