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/main.rs

@@ -1333,6 +1333,10 @@ async fn scan_one(
     // The body the scan already fetched for it, if it did (#104).
     prefetched: Option<Result<feed::Fetched>>,
 ) -> Result<Outcome> {
+    // The queue follows the feed's settings each time it is due, changed or not, so files a
+    // limit or auto-download no longer reaches stop counting as waiting (#113).
+    let policy = policy_for(ctx, id, feed_cfg).await?;
+    ctx.db.hold_back(id, if policy.auto_download { policy.budget } else { 0 }).await?;
     // A Patreon creator with more than one show is a list of feeds, like an OPML.
     if feed::is_patreon_creator(&feed_cfg.url) {
         match feed::patreon_shows(&ctx.client, &feed_cfg.url).await {
@@ -1456,7 +1460,6 @@ async fn scan_one(
         parsed.category.as_deref(),
     ).await?;
 
-    let policy = policy_for(ctx, id, feed_cfg).await?;
     if let Some(parent) = &feed_cfg.group {
         let listed: Vec<(&str, &str)> = parsed
             .entries
@@ -1503,6 +1506,9 @@ async fn scan_one(
     if scan.new_entries > 0 {
         ctx.db.rehide(id).await?;   // what is new may hold someone's blocked words
     }
+    // A new item takes a place among the newest and the oldest of them leaves the queue; a file a
+    // filter lets through again may be outside them.
+    ctx.db.hold_back(id, if policy.auto_download { policy.budget } else { 0 }).await?;
 
     let budget = policy.budget;
     if policy.auto_download && budget > 0 {