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/db.rs

@@ -415,6 +415,8 @@ pub struct HttpState {
     pub last_modified: Option<String>,
     pub last_checked: Option<i64>,
     pub ttl_mins: Option<u64>,
+    /// When the feed's current run of failures began, for backing off (`due_after`).
+    pub error_since: Option<i64>,
 }
 
 impl Db {
@@ -427,6 +429,7 @@ impl Db {
                 last_modified: f.last_modified,
                 last_checked: f.last_checked,
                 ttl_mins: f.ttl_mins.map(|t| t.max(0) as u64),
+                error_since: f.error_since,
             })
             .unwrap_or_default())
     }