Forget a failing feed once nobody subscribes to it (#100)

Unsubscribing leaves a feed's row and history, which suits one that worked. One that never did
stayed with its error for good and was never scanned again: cnn-com, added as a bare 'cnn.com'
(#101), sat there failing with no subscriber. The reaper, before each scan, now deletes a feed
that is failing, has no subscriber, is not in the catalogue and has no file on disk, with its
items, file rows, read state and block list.

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

src/retention.rs

@@ -57,6 +57,10 @@ pub async fn run(cfg: &Config, db: &Db, dry_run: bool) -> Result<Report> {
         report.reconciled += 1;
     }
 
+    for id in db.prune_abandoned_failures().await? {
+        tracing::info!(feed = id, "forgot a failing feed nobody subscribes to");
+    }
+
     let candidates = db.reap_candidates().await?;
 
     if cfg.general.max_age_days > 0 {