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_sync_guards.py

@@ -1,5 +1,6 @@
 """Guards that stop one bad YouTube answer from cascading: the subscription prune guard, the
 playlist-push circuit breaker, and RSS telling a failure apart from an empty feed."""
+
 import httpx
 import pytest
 
@@ -176,9 +177,7 @@ class TestRssPollResult:
 
     def test_no_channels_at_all_is_not_an_outage(self, monkeypatch):
         # A fresh instance with nothing subscribed must not report an error every poll.
-        out = self._run(
-            monkeypatch, fetch=lambda cid: [], apply_=lambda db, ch, e: 0, channels=()
-        )
+        out = self._run(monkeypatch, fetch=lambda cid: [], apply_=lambda db, ch, e: 0, channels=())
         assert out == {"new": 0, "failed": 0}