fix(downloads): a conversion's commit, a stopped upload, the fast join and a clip's copy hold up

- The fast join gives each ffconcat entry its own media-only format whitelist, so a library file
  can no longer be opened as a nested concat list (the list's whitelist was handed down to it).
- A file the guarded probe cannot read is a download-only file, not a guess from yt-dlp's info.
- A conversion's or an ingest's commit that raises is re-read in a fresh session before any file
  is removed: a commit that landed keeps the file its row now names.
- An original that cannot be removed after conversion is logged; the upload stays ready.
- An upload whose conversion is cancelled or deleted midway is freed once nothing holds it; a
  paused one is kept.
- A shared link says "preparing" only while its video's job is queued or running.
- The admin storage view counts every file on disk ("Files on disk"), not only ready ones.
- A clip's failure title, unknown failure and cancel dialog, and a clip's and an upload's delete
  dialog speak of what the job made; an upload's quota refusal explains that the original is kept.
- /watch announces "preparing" as an alert and moves focus to the new Retry after an action.

backend/tests/test_ffmpeg_input_guard.py

@@ -75,10 +75,45 @@ def test_a_join_is_guarded(accurate, tmp_path):
     segs = [{"start_s": 0, "end_s": 1}, {"start_s": 2, "end_s": 3}]
     spec = edit.normalize_edit_spec({"segments": segs, "accurate": accurate})
     cmd, _ = edit.build_concat_plan(Path("in.mp4"), Path("out.mp4"), spec, "mp4", tmp_path)
-    # The fast join reads a concat list WE write; the whitelist rides down to the file it names.
+    # The fast join reads a concat list WE write, so the list itself may be opened as concat.
     _assert_media_guarded(cmd, extra=set() if accurate else {"concat"})
 
 
+def test_the_fast_join_list_gives_each_entry_the_media_list_without_concat(tmp_path):
+    # ffmpeg hands the list's whitelist (with `concat`) down to the library file each entry names;
+    # the entry's own `option format_whitelist` must replace it, or that file may open as a list.
+    segs = [{"start_s": 0, "end_s": 1}, {"start_s": 2, "end_s": 3}]
+    spec = edit.normalize_edit_spec({"segments": segs, "accurate": False})
+    _, prep = edit.build_concat_plan(Path("in.mp4"), Path("out.mp4"), spec, "mp4", tmp_path)
+    (text,) = prep.values()
+    lines = text.splitlines()
+    entries = [i for i, ln in enumerate(lines) if ln.startswith("file ")]
+    assert len(entries) == 2
+    for i in entries:
+        key, _, allowed = lines[i + 1].partition(" format_whitelist ")
+        assert key == "option", lines[i + 1]
+        assert set(allowed.split(",")) == MEDIA
+
+
+@pytest.mark.skipif(shutil.which("ffmpeg") is None, reason="ffmpeg not available")
+def test_the_fast_join_still_joins_a_real_file(tmp_path):
+    src = tmp_path / "in.mp4"
+    subprocess.run(
+        ["ffmpeg", "-y", "-loglevel", "error", "-f", "lavfi", "-i", "testsrc=size=64x48:rate=10",
+         "-f", "lavfi", "-i", "sine=frequency=440", "-t", "4", "-c:v", "libx264",
+         "-pix_fmt", "yuv420p", "-c:a", "aac", str(src)],
+        check=True,
+    )  # fmt: skip
+    segs = [{"start_s": 0, "end_s": 1}, {"start_s": 2, "end_s": 3}]
+    spec = edit.normalize_edit_spec({"segments": segs, "accurate": False})
+    out = tmp_path / "out.mp4"
+    cmd, prep = edit.build_concat_plan(src, out, spec, "mp4", tmp_path)
+    for p, content in prep.items():
+        p.write_text(content, encoding="utf-8")
+    subprocess.run(cmd, check=True, capture_output=True)
+    assert (edit.probe_duration(out) or 0) > 0
+
+
 def test_the_poster_and_the_filmstrip_are_guarded():
     _assert_media_guarded(edit.build_poster_cmd(Path("in.mp4"), Path("p.jpg"), 2.0))
     geom = edit.storyboard_geometry(60)