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>

src/web.rs

@@ -790,11 +790,11 @@ async fn feeds(
                 .schedule
                 .as_deref()
                 .and_then(crate::config::parse_interval),
-            every_mins: crate::due_after(&cfg, feed, ttl_mins) / 60,
+            every_mins: crate::due_after(&cfg, feed, ttl_mins, None, 0) / 60,
             last_checked: s.last_checked,
             next_check: s
                 .last_checked
-                .map(|t| t + crate::due_after(&cfg, feed, ttl_mins) as i64),
+                .map(|t| t + crate::due_after(&cfg, feed, ttl_mins, s.error_since, t) as i64),
             failing: s
                 .error_since
                 .filter(|since| crate::db::now() - since >= FLAG_AFTER_SECS)