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/auth/handlers.rs

@@ -39,6 +39,7 @@ pub async fn join(
     if !state.rate_limiter.check(format!("join:{ip}"), 5, Duration::from_secs(60)) {
         return Err(AppError::TooManyRequests(
             "Zu viele Anfragen. Bitte warte kurz und versuche es erneut.".into(),
+            None,
         ));
     }
 
@@ -124,6 +125,7 @@ pub async fn recover(
             if Utc::now() < locked_until {
                 return Err(AppError::TooManyRequests(
                     "Zu viele Versuche. Bitte warte 15 Minuten.".into(),
+                    None,
                 ));
             }
         }