feat: auto-retry uploads when rate limited

Backend: extend rate limiter with check_with_retry() that returns the
seconds until the next slot opens. Upload 429 responses now include
retry_after_secs in the JSON body and a Retry-After header.

Frontend: the upload queue catches 429s as RateLimitError, resets the
affected item to pending, schedules processQueue() to resume after the
server-reported wait, and shows a live countdown banner in UploadQueue.

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

backend/src/handlers/upload.rs

@@ -20,13 +20,14 @@ pub async fn upload(
 ) -> Result<(StatusCode, Json<UploadDto>), AppError> {
     // Rate limit: N uploads per hour per user
     let upload_rate = get_config_i64(&state.pool, "upload_rate_per_hour", 10).await as usize;
-    if !state
+    if let Err(retry_after_secs) = state
         .rate_limiter
-        .check(format!("upload:{}", auth.user_id), upload_rate, Duration::from_secs(3600))
+        .check_with_retry(format!("upload:{}", auth.user_id), upload_rate, Duration::from_secs(3600))
     {
         drain_multipart(multipart).await;
         return Err(AppError::TooManyRequests(
             "Du hast dein Upload-Limit für diese Stunde erreicht.".into(),
+            Some(retry_after_secs),
         ));
     }