test(6849): the restore command previews without --yes and rolls back from the tar with it

Two integration tests in test_integration_backup_rollback.py drive
grimoire.restore.run on the scheduled backup's tar, written from a full
export of this instance:
- without --yes it exits 4 and nothing changes;
- with --yes it takes the safety backup, then puts back the edited and the
  deleted task and removes the new one.

The file-restore test that expected a refusal "because the database has
accounts" now names the reason it actually refuses: on a live instance, a
backup that records no instance id can't be rolled back to.

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

tests/test_integration_backup_file_restore.py

@@ -5,7 +5,8 @@ the real `restore_full_backup`.
 The unit lane already shows the tar hands back the dict it was given. What
 only this lane shows is that the dict it hands back is one the restore
 accepts — every id remapped to the restored rows, nothing dropped — and that
-the command-line restore refuses a database that already has accounts.
+the command-line restore refuses, on a live instance, a backup that doesn't
+record this instance as its source.
 """
 import pytest
 import pytest_asyncio
@@ -97,10 +98,11 @@ async def test_the_tar_restores_with_every_id_remapped(written):
     assert note.status == "todo"
 
 
-async def test_the_command_refuses_a_database_with_accounts(written):
-    """The source user exists, so this database is not empty: the command
-    must stop before restoring anything."""
-    assert await run(written) == 3
+async def test_the_command_refuses_a_backup_from_no_known_instance(written):
+    """This database is live and the backup names no instance it came from,
+    so it can't be rolled back to: the command stops before restoring
+    anything, even with --yes."""
+    assert await run(written, yes=True) == 3
     async with async_session() as s:
         restored = (await s.execute(select(User).where(User.username == RESTORED))).scalars().all()
     assert restored == []