fix: a restore with pg_restore 17 into Postgres 16 isn't stopped by a setting the server lacks

pg_restore 17 opens with `SET transaction_timeout = 0`, which Postgres 16 refuses.
Under --exit-on-error that one harmless statement ended every restore: the
image ships pg_restore 17.11 and the bundled server is pg16, so a restore
from the bucket (and the local restore) could never have loaded a dump. The
backup round trip, now that CI has pg_dump, found it.

pg_restore now runs to the end and its errors are judged afterwards: the
known harmless one is ignored, and any other fails the restore with that
error's words rather than the "Command was:" line that followed it.

Also sorts an import the lint lane flagged.

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

tests/test_backup_restore.py

@@ -198,7 +198,7 @@ def test_a_restore_out_of_time_queues_its_next_chunk(db_sync, tmp_path, run_env,
 def test_a_restart_resumes_the_file_phase_and_drops_the_stale_chunk(
     db_sync, tmp_path, run_env,
 ):
-    from backend.app.tasks.backup import resume_offsite_runs, restore_offsite_task
+    from backend.app.tasks.backup import restore_offsite_task, resume_offsite_runs
     _file(tmp_path, "a.jpg", b"aa")
     _image(db_sync, tmp_path / "a.jpg", b"aa")
     now = datetime.now(UTC)