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>

CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- A feed that was failing when its last subscriber left is forgotten, rather than kept with its error for good. One that worked, or has files on disk, is kept as before.
 - A feed that keeps failing is checked less and less often, waiting as long as it has been failing, up to once a day; it goes back to its schedule as soon as it works. Refreshing it still checks it at once.
 - A scan's trace shows the time a feed spends on its artwork and in the database after the fetch.
 - The JSON log carries each line's `trace_id` and `span_id`, logs each scan event once instead of