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>

docs/configuration.md

@@ -45,7 +45,9 @@ media_types       = ["audio", "video"]
 database as described above; `download_dir`, `socket` and `organize` stay in config.toml.
 
 * **`schedule`** — how often feeds are re-checked. A feed's own `<ttl>` still wins when it asks to
-  be polled *less* often, and a per-feed `schedule` overrides both. Admin-only from the UI.
+  be polled *less* often, and a per-feed `schedule` overrides both. A feed that keeps failing
+  waits as long as it has been failing before the next try, up to a day, and is back on schedule
+  after its first success. Admin-only from the UI.
 * **`organize`** — `feed` files downloads under the feed's folder; `date` under `YYYY-MM-DD`.
 * **`max_total_gb`** — the reaper deletes to get back under this, oldest first, keeping a 50 MB
   pad. Kept items are never deleted, and a file only counts as read once every subscriber has