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_worker_supervisor.py

@@ -4,6 +4,7 @@ the DB-liveness lookup + side effects are monkeypatched, so the decision logic i
 machine. These are regression tests for the class of bug that motivated the whole change: a wedged
 download that a thread could neither kill nor cancel.
 """
+
 import signal
 import subprocess
 
@@ -45,6 +46,7 @@ def rec(monkeypatch):
     def record(name):
         def _f(*args, **kwargs):
             calls.append((name, args, kwargs))
+
         return _f
 
     monkeypatch.setattr(worker, "_reset_asset_pending", record("reset_asset"))
@@ -209,6 +211,7 @@ def test_a_watchdog_error_still_stops_the_child(monkeypatch, rec):
 
 # --- _stop_child escalation -----------------------------------------------------------------
 
+
 @pytest.fixture
 def killpg_rec(monkeypatch):
     sent = []