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>

frontend/src/lib/components/UploadQueue.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-	import { queueItems, isProcessing, retryItem, removeItem, clearCompleted } from '$lib/upload-queue';
+	import { queueItems, isProcessing, retryItem, removeItem, clearCompleted, rateLimitRetryAt } from '$lib/upload-queue';
 	import type { QueueItem } from '$lib/upload-queue';
 
 	function formatSize(bytes: number): string {
@@ -28,6 +28,25 @@
 
 	let items = $derived($queueItems);
 	let hasCompleted = $derived(items.some((i) => i.status === 'done'));
+
+	// Countdown for rate-limit banner
+	let countdown = $state(0);
+
+	$effect(() => {
+		const retryAt = $rateLimitRetryAt;
+		if (!retryAt) {
+			countdown = 0;
+			return;
+		}
+
+		countdown = Math.ceil((retryAt - Date.now()) / 1000);
+		const interval = setInterval(() => {
+			countdown = Math.ceil((retryAt - Date.now()) / 1000);
+			if (countdown <= 0) clearInterval(interval);
+		}, 1000);
+
+		return () => clearInterval(interval);
+	});
 </script>
 
 {#if items.length > 0}
@@ -49,6 +68,12 @@
 			{/if}
 		</div>
 
+		{#if $rateLimitRetryAt && countdown > 0}
+			<div class="border-b border-amber-100 bg-amber-50 px-4 py-2 text-sm text-amber-800">
+				Upload-Limit erreicht. Wird in {countdown} Sek. automatisch fortgesetzt.
+			</div>
+		{/if}
+
 		<ul class="divide-y divide-gray-100">
 			{#each items as item (item.id)}
 				<li class="px-4 py-3">