fix(playlists): "+N without length" counts every video the sum left out

The label said "without length" but the count held only live videos, so
a video not fetched yet, or gone before it was, was left out of the sum
and named nowhere. The server's `Video.without_length` and the client's
`isWithoutLength` are now one rule (no length, whatever the live
status), and the field, the key and the helpers are named for what they
count. The comments that still quoted "+N live" say the current copy.

backend/app/routes/playlists.py

@@ -99,9 +99,9 @@ def _summary(
     has_video: bool | None = None,
     cover: str | None = None,
 ) -> dict:
-    """`duration` is `queries.durations`'s pair: the known running time, and the live videos it
-    cannot hold — the "+N live" beside it."""
-    total_duration, live = duration
+    """`duration` is `queries.durations`'s pair: the known running time, and the videos it left out
+    for having no length — the "+N without length" beside it."""
+    total_duration, left_out = duration
     out = {
         "id": pl.id,
         "name": pl.name,
@@ -111,7 +111,7 @@ def _summary(
         "dirty": pl.dirty,
         "item_count": count,
         "total_duration_seconds": total_duration,
-        "live_without_length": live,
+        "without_length": left_out,
         "cover_thumbnail": cover,
         # When this list last CHANGED (rename, add/remove/move/reorder) — the rail's
         # "recently changed first" sort reads it. See `service.touch` for why it isn't automatic.
@@ -369,8 +369,8 @@ def rename_playlist(
             raise _refused(400, "name_required")
         service.rename_playlist(db, pl, name)
     db.commit()
-    count, total, live, cover = queries.playlist_totals(db, pl.id)
-    return _summary(pl, count, (total, live), cover=cover)
+    count, total, left_out, cover = queries.playlist_totals(db, pl.id)
+    return _summary(pl, count, (total, left_out), cover=cover)
 
 
 @router.delete("/{playlist_id}")