feat: auto-retry uploads when rate limited (v0.13.0)

Backend: rate limiter gains check_with_retry() returning seconds until
the next slot opens. Upload 429 responses include retry_after_secs in
JSON and a Retry-After header.

Frontend: upload queue catches 429 as RateLimitError, resets affected
item to pending, schedules processQueue() for the server-reported delay,
and shows a live countdown banner in the queue UI.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/handlers/feed.rs

@@ -63,9 +63,7 @@ pub async fn feed(
     let ip = client_ip(&headers, "unknown");
     let rate_limit = get_config_usize(&state.pool, "feed_rate_per_min", 60).await;
     if !state.rate_limiter.check(format!("feed:{ip}"), rate_limit, Duration::from_secs(60)) {
-        return Err(AppError::TooManyRequests(
-            "Zu viele Anfragen. Bitte warte kurz und versuche es erneut.".into(),
-        ));
+        return Err(AppError::TooManyRequests("Zu viele Anfragen. Bitte warte kurz und versuche es erneut.".into(), None));
     }
 
     let limit = q.limit.unwrap_or(20).min(100);