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_plex_stream.py

@@ -6,6 +6,7 @@ that keeps the sweep off a neighbour's data, so the names it accepts are pinned
 
 Since the sweep stopped being awaited at startup it also runs CONCURRENTLY with playback, so the
 second half of this file pins what keeps it off a LIVE session's segments."""
+
 import os
 
 import pytest