style: apply ruff format across backend + scripts (line-length 100)

Mechanical, whole-tree `ruff format` pass — no behaviour change. Recorded in
.git-blame-ignore-revs (next commit) so git blame skips it.

backend/tests/test_db_lane.py

@@ -1,6 +1,7 @@
 """Smoke test proving the DB-backed lane works (R6 S3b): a real commit round-trips, and each
 test starts from a clean database (TRUNCATE isolation), which is what lets the transaction-
 ownership tests (R6 S2) trust what they commit."""
+
 from sqlalchemy import func, select
 
 from app.models import User