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/app/plex/imgcache.py

@@ -63,6 +63,4 @@ def write(key: str, data: bytes, ct: str) -> None:
 
 
 def response(data: bytes, ct: str) -> Response:
-    return Response(
-        content=data, media_type=ct, headers={"Cache-Control": "public, max-age=86400"}
-    )
+    return Response(content=data, media_type=ct, headers={"Cache-Control": "public, max-age=86400"})