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_thumbnail_poster.py

@@ -5,6 +5,7 @@ already-JPEG thumbnails (Reddit serves a JPEG under a `.png` name) — and becau
 error is fatal, it used to fail the WHOLE download. The key regression property here is that a bad
 thumbnail returns None (poster falls back to a cut frame) instead of raising.
 """
+
 import shutil
 import subprocess
 from pathlib import Path
@@ -25,8 +26,19 @@ def _is_jpeg(p: Path) -> bool:
 def test_converts_a_png_to_a_real_jpeg(tmp_path):
     src = tmp_path / "thumb.png"
     subprocess.run(
-        ["ffmpeg", "-y", "-loglevel", "error", "-f", "lavfi",
-         "-i", "color=c=red:s=64x64", "-frames:v", "1", str(src)],
+        [
+            "ffmpeg",
+            "-y",
+            "-loglevel",
+            "error",
+            "-f",
+            "lavfi",
+            "-i",
+            "color=c=red:s=64x64",
+            "-frames:v",
+            "1",
+            str(src),
+        ],
         check=True,
     )
     out = edit.thumbnail_to_jpg(src, tmp_path / "poster.jpg")