Back off a failing feed exponentially, up to a day (#99)

A feed that failed was tried again on its usual schedule however long it had been failing:
gizmodo's 404, pelgrane's 403, daily-quests' 503 and toddstashwick's redirect loop every hour,
each a request to a site that had said no, a warning and scan time. A failing feed now waits as
long as it has been failing, from error_since to its last check, never less than its usual
interval and never more than a day: 1h, 1h, 2h, 4h, 8h, 16h, then daily on an hourly schedule.
No new column: error_since already marks the run's start and the first success clears it. A
forced refresh skips the due check, so it still tries at once. The feed list's next check
follows the backoff.

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 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
   twice, names a failure's kind in `error.type` (and its HTTP status in