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>

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">

frontend/src/lib/upload-queue.ts

@@ -18,6 +18,9 @@ export interface QueueItem {
 export const queueItems = writable<QueueItem[]>([]);
 export const isProcessing = writable(false);
 
+/** Set to the timestamp (ms) at which the rate-limit lifts, or null when clear. */
+export const rateLimitRetryAt = writable<number | null>(null);
+
 const DB_NAME = 'eventsnap-uploads';
 const STORE_NAME = 'queue';
 
@@ -35,6 +38,14 @@ async function getDb(): Promise<IDBPDatabase> {
 	return db;
 }
 
+class RateLimitError extends Error {
+	retryAfterSecs: number;
+	constructor(secs: number) {
+		super('rate_limited');
+		this.retryAfterSecs = secs;
+	}
+}
+
 export async function loadQueue(): Promise<void> {
 	const database = await getDb();
 	const all = await database.getAll(STORE_NAME);
@@ -136,7 +147,21 @@ async function processQueue(): Promise<void> {
 			const next = items.find((item) => item.status === 'pending');
 			if (!next) break;
 
-			await uploadItem(next.id);
+			try {
+				await uploadItem(next.id);
+			} catch (e) {
+				if (e instanceof RateLimitError) {
+					// Keep all pending items as-is; schedule queue resume when limit lifts
+					const retryAt = Date.now() + e.retryAfterSecs * 1000;
+					rateLimitRetryAt.set(retryAt);
+					setTimeout(() => {
+						rateLimitRetryAt.set(null);
+						processQueue();
+					}, e.retryAfterSecs * 1000);
+					break;
+				}
+				// Other errors are already handled inside uploadItem (marked as 'error')
+			}
 		}
 	} finally {
 		processing = false;
@@ -148,7 +173,6 @@ async function uploadItem(id: string): Promise<void> {
 	const database = await getDb();
 	const entry = await database.get(STORE_NAME, id);
 	if (!entry || !entry.blob) {
-		// No blob — mark as error
 		updateItemStatus(id, 'error', 'Datei nicht gefunden.');
 		return;
 	}
@@ -184,6 +208,14 @@ async function uploadItem(id: string): Promise<void> {
 			xhr.addEventListener('load', () => {
 				if (xhr.status >= 200 && xhr.status < 300) {
 					resolve();
+				} else if (xhr.status === 429) {
+					try {
+						const body = JSON.parse(xhr.responseText);
+						const secs = typeof body.retry_after_secs === 'number' ? body.retry_after_secs : 60;
+						reject(new RateLimitError(secs));
+					} catch {
+						reject(new RateLimitError(60));
+					}
 				} else {
 					try {
 						const body = JSON.parse(xhr.responseText);
@@ -205,6 +237,13 @@ async function uploadItem(id: string): Promise<void> {
 		await database.put(STORE_NAME, entry);
 		updateItemStatus(id, 'done');
 	} catch (e) {
+		if (e instanceof RateLimitError) {
+			// Reset to pending so it will be retried when the queue resumes
+			entry.status = 'pending';
+			await database.put(STORE_NAME, entry);
+			updateItemStatus(id, 'pending');
+			throw e; // Propagate to processQueue for scheduling
+		}
 		const msg = e instanceof Error ? e.message : 'Upload fehlgeschlagen.';
 		entry.status = 'error';
 		entry.error = msg;
@@ -224,7 +263,7 @@ function updateItemStatus(
 				? {
 						...item,
 						status,
-						progress: status === 'done' ? 100 : status === 'error' ? item.progress : item.progress,
+						progress: status === 'done' ? 100 : status === 'pending' ? 0 : item.progress,
 						error
 					}
 				: item