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_ffmpeg_callbacks.py

@@ -4,6 +4,7 @@ run_ffmpeg calls `on_progress`/`should_cancel` once per ffmpeg output LINE (`-pr
 emits a block of ~12 every half second), and both callbacks are database round-trips: `_set_job`
 opens a session and UPDATE+COMMITs, `_job_status` opens one and SELECTs. Unthrottled, a long
 re-encode is ~10^5 of each."""
+
 import app.downloads.pipeline as pipeline
 
 
@@ -71,7 +72,9 @@ class TestProgressWriter:
         write = pipeline._ffmpeg_progress_writer(7, "processing", now=lambda: clock["t"])
         write(6.0)  # progress write at t=1000 → refreshes updated_at
         clock["t"] += 0.3
-        write(6.4)  # same percent, SAME interval → the write already touched the row, so no heartbeat
+        write(
+            6.4
+        )  # same percent, SAME interval → the write already touched the row, so no heartbeat
         assert [w["progress"] for w in writes] == [6]
         assert touches == []
 
@@ -79,7 +82,9 @@ class TestProgressWriter:
 class TestCancelPoll:
     def test_asks_once_then_reuses_the_answer(self, monkeypatch):
         calls = []
-        monkeypatch.setattr(pipeline, "_job_status", lambda job_id: calls.append(job_id) or "running")
+        monkeypatch.setattr(
+            pipeline, "_job_status", lambda job_id: calls.append(job_id) or "running"
+        )
         cancelled = pipeline._cancel_poll(7)
         for _ in range(40):
             assert cancelled() is False