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/entity.rs

@@ -102,6 +102,10 @@ pub mod enclosures {
         pub length: Option<i64>,
         #[sea_orm(column_type = "Text", nullable)]
         pub path: Option<String>,
+        /// 'pending' is queued: a scan downloads it on its own. 'held' is listed but outside the
+        /// feed's newest max_new_per_check, or its feed downloads nothing automatically; it can
+        /// be downloaded by hand (`Db::hold_back`). 'skipped' is filtered out ('last_error' says
+        /// why), then 'downloading', 'done', 'error', 'reaped'.
         #[sea_orm(column_type = "Text")]
         pub state: String,
         #[sea_orm(default_value = 0)]