Count as queued only what a scan will download on its own (#113)

Every file in state 'pending' counted as waiting to download: 4420 in production, across 12
shows. Since #97 a scan only downloads among a feed's newest max_new_per_check items, so those
were back-catalogue episodes no scan would take; the real queue was 0.

A new state, 'held': listed and downloadable by hand, but outside the feed's newest items, or of
a feed nothing downloads automatically. Db::hold_back moves a feed's waiting files between
'pending' and 'held' each time the feed is due, changed or not, and again after its items are
stored, so a new episode, a raised limit or auto-download turned on or off moves them. A held
file keeps its item's place among the newest, as a downloaded one does. 'pending' now means
queued, so ipx status, /api/status and the dashboard read true without changing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ipc.rs

@@ -69,7 +69,7 @@ impl Event {
                 *bytes as f64 / 1_048_576.0
             ),
             Event::Status { feeds, pending, downloaded } => {
-                format!("{feeds} feeds, {pending} pending, {downloaded} downloaded")
+                format!("{feeds} feeds, {pending} queued to download, {downloaded} downloaded")
             }
             Event::Error { msg } => format!("error: {msg}"),
             // Noise in a terminal; a UI still gets them on the socket.