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/app/downloads/edit.py

@@ -338,13 +338,16 @@ def build_concat_plan(src: Path, dest: Path, spec: dict, out_ext: str, staging:
         ]
     else:
         listp = staging / "concat.txt"
-        lines = []
+        lines = ["ffconcat version 1.0"]
         for s in segs:
             lines.append(f"file '{_concat_quote(src)}'")
+            # ffmpeg hands the list's whitelist (which must name `concat` to read the list) down to
+            # the file it names, so a library file whose bytes are a concat list would be read as one
+            # and open the paths inside it. Each entry's own whitelist replaces the inherited one.
+            lines.append(f"option format_whitelist {INPUT_GUARD[1]}")
             lines.append(f"inpoint {s['start_s']}")
             lines.append(f"outpoint {s['end_s']}")
         prep[listp] = "\n".join(lines) + "\n"
-        # The list is ours; the whitelist reaches the file it names too (ffmpeg hands it down).
         guard = ["-format_whitelist", "concat," + INPUT_GUARD[1]]
         cmd = head + ["-f", "concat", "-safe", "0", *guard, "-i", str(listp), "-c", "copy"]