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_utils.py

@@ -1,4 +1,5 @@
 """iso(): the single date/datetime→JSON serializer the routes share (R6 S1a)."""
+
 from datetime import date, datetime, timezone
 
 from app.utils import iso, normalize_description