fix(editor): a crop of a video with wide pixels keeps what its frame showed

The browser reports such a video by its shown size and ffmpeg's crop counts stored pixels, so a
frame over the middle half of a 320 stored, 640 shown video kept the whole width. A crop now carries
the size it was measured on and is scaled to the file's own pixels. A spec from before it did is cut
as it always was. Also drops a stale note in downloads/quota.py: edit jobs are held to the cap.

backend/tests/test_edit_crop_frame.py

@@ -0,0 +1,112 @@
+"""The editor's crop lands on the part of the picture the frame was drawn on, also when a video's
+pixels are not square. The browser reports such a video by its shown size (320 stored, 640 shown),
+and ffmpeg's `crop` counts stored pixels: measured on a real clip, a frame over the middle half kept
+half the frame's red surroundings. So a crop carries the size it was measured on.
+
+Real ffmpeg: the pixels of the output are the oracle.
+"""
+
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+from app.config import settings
+from app.downloads import edit, pipeline, service, storage
+from app.downloads import upload as uploadmod
+from app.downloads.models import DownloadJob, MediaAsset
+
+pytestmark = pytest.mark.skipif(shutil.which("ffmpeg") is None, reason="ffmpeg not available")
+
+
+def _source(path: Path, size: str, box: str, sar: int) -> None:
+    """Red, with the picture's middle half green: a crop of that middle holds no red."""
+    subprocess.run(
+        [
+            "ffmpeg", "-y", "-loglevel", "error",
+            "-f", "lavfi", "-i", f"color=c=red:s={size}:d=2:r=10",
+            "-f", "lavfi", "-i", "sine=duration=2",
+            "-vf", f"drawbox={box}:c=lime:t=fill,setsar={sar}",
+            "-c:v", "libx264", "-pix_fmt", "yuv420p", "-c:a", "aac", str(path),
+        ],
+        check=True,
+    )  # fmt: skip
+
+
+def _red_and_green(path: Path) -> tuple[int, int]:
+    raw = subprocess.run(
+        ["ffmpeg", "-loglevel", "error", "-i", str(path), "-frames:v", "1",
+         "-f", "rawvideo", "-pix_fmt", "rgb24", "-"],
+        check=True,
+        capture_output=True,
+    ).stdout  # fmt: skip
+    red = green = 0
+    for i in range(0, len(raw) - 2, 3):
+        if raw[i] > 180 and raw[i + 1] < 80:
+            red += 1
+        elif raw[i + 1] > 180 and raw[i] < 80:
+            green += 1
+    return red, green
+
+
+def _cut(db, make_user, tmp_path, src: Path, crop: dict) -> Path:
+    user = make_user()
+    source = uploadmod.ingest_upload(
+        db, user.id, filename="clip.mp4", src_path=src, size=src.stat().st_size
+    )
+    assert source.status == "done"
+    job = service.enqueue_edit(db, user.id, source, {"crop": crop})
+    asset = db.get(MediaAsset, job.asset_id)
+    job.status, asset.status = "running", "downloading"  # the worker's claim
+    db.commit()
+
+    pipeline._process_edit(job.id, asset.id)
+
+    db.expire_all()
+    job, asset = db.get(DownloadJob, job.id), db.get(MediaAsset, job.asset_id)
+    assert (job.status, job.error, asset.status) == ("done", None, "ready")
+    return storage.abs_path(str(tmp_path), asset.rel_path)
+
+
+def test_a_crop_of_a_video_with_wide_pixels_keeps_what_the_frame_showed(
+    db, make_user, tmp_path, monkeypatch
+):
+    monkeypatch.setattr(settings, "files_root", str(tmp_path))
+    src = tmp_path / "in.mp4"
+    _source(src, "320x240", "x=80:y=60:w=160:h=120", sar=2)
+
+    # What the editor sends for the middle half of this video: the browser shows it at 640 x 240.
+    final = _cut(
+        db, make_user, tmp_path, src,
+        {"x": 160, "y": 60, "w": 320, "h": 120, "frame_w": 640, "frame_h": 240},
+    )  # fmt: skip
+
+    red, green = _red_and_green(final)
+    assert (red, green) == (0, 160 * 120)
+
+
+def test_a_crop_of_square_pixels_is_cut_where_it_was_drawn(db, make_user, tmp_path, monkeypatch):
+    monkeypatch.setattr(settings, "files_root", str(tmp_path))
+    src = tmp_path / "in.mp4"
+    _source(src, "480x640", "x=120:y=160:w=240:h=320", sar=1)
+
+    final = _cut(
+        db, make_user, tmp_path, src,
+        {"x": 120, "y": 160, "w": 240, "h": 320, "frame_w": 480, "frame_h": 640},
+    )  # fmt: skip
+
+    assert _red_and_green(final) == (0, 240 * 320)
+
+
+def test_a_crop_stored_before_it_carried_its_frame_is_cut_as_it_always_was():
+    spec = edit.normalize_edit_spec({"crop": {"x": 4, "y": 6, "w": 100, "h": 50}})
+    assert spec["crop"] == {"x": 4, "y": 6, "w": 100, "h": 50}
+    cmd = edit.build_edit_cmd(Path("in.mp4"), Path("out.mp4"), spec, "mp4")
+    assert cmd[cmd.index("-vf") + 1].startswith("crop=100:50:4:6,")
+
+
+@pytest.mark.parametrize("frame", [{"frame_w": 0, "frame_h": 240}, {"frame_w": "x", "frame_h": 2}])
+def test_a_frame_that_cannot_be_read_is_not_divided_by(frame):
+    spec = edit.normalize_edit_spec({"crop": {"x": 4, "y": 6, "w": 100, "h": 50, **frame}})
+    assert spec["crop"] == {"x": 4, "y": 6, "w": 100, "h": 50}