Clamp an unlimited download queue's LIMIT for Postgres (#98)

With max_new_per_check at 0 and no per-subscription limit, the budget is usize::MAX, and
pending() bound it `as i64`: -1. SQLite reads LIMIT -1 as no limit; Postgres refuses it, so a
feed's downloads failed. The new test fails with "LIMIT must not be negative" on Postgres
without the clamp and passes with it.

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

src/db.rs

@@ -635,7 +635,9 @@ impl Db {
              WHERE x.feed_id = $1 AND x.state = 'pending'
              ORDER BY coalesce(e.published, e.first_seen) DESC, x.id DESC
              LIMIT $2",
-            vec![feed_id.into(), (limit as i64).into()],
+            // Unlimited is usize::MAX, which `as i64` makes -1: SQLite reads LIMIT -1 as no
+            // limit, Postgres refuses it, and the feed's downloads failed (#98).
+            vec![feed_id.into(), (limit.min(i64::MAX as usize) as i64).into()],
         )
         .await?
         .iter()
@@ -1934,6 +1936,17 @@ mod tests {
         assert_eq!((list["f"].unread, list["g"].unread), (1, 1)); // a read, c hidden; sam's read is sam's
     }
 
+    #[tokio::test]
+    async fn an_unlimited_queue_takes_everything_waiting() {
+        let db = Db::memory().await.unwrap();
+        db.exec_for_test(
+            "INSERT INTO entries (feed_id, guid, first_seen) VALUES ('f','a',0),('f','b',0);
+             INSERT INTO enclosures (id, feed_id, guid, url, state) VALUES (1,'f','a','u1','pending'),(2,'f','b','u2','pending');",
+        ).await
+        .unwrap();
+        assert_eq!(db.pending("f", usize::MAX).await.unwrap().len(), 2);
+    }
+
     #[tokio::test]
     async fn a_scan_knows_a_feeds_stored_items_and_files() {
         let db = Db::memory().await.unwrap();