feat(frontend): add admin card to generate thumbnails with polling

Fourth card on /admin/system mirrors the mass-import pattern:
- POST /api/admin/generate-thumbnails to trigger
- 2000 ms polling on /api/admin/thumbnail-status while RUNNING
- processed / skipped / failed counters in the DONE message
- standalone pollInterval so import and thumbnail polling don't
  interfere with each other

Paraglide keys added in de/en/es, mirroring admin_system_import_*.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/routes/admin/system/+page.svelte

@@ -14,8 +14,20 @@ type ImportStatus = {
 	startedAt: string | null;
 };
 
+type ThumbnailStatus = {
+	state: 'IDLE' | 'RUNNING' | 'DONE' | 'FAILED';
+	message: string;
+	total: number;
+	processed: number;
+	skipped: number;
+	failed: number;
+	startedAt: string | null;
+};
+
 let importStatus: ImportStatus | null = $state(null);
+let thumbnailStatus: ThumbnailStatus | null = $state(null);
 let pollInterval: ReturnType<typeof setInterval> | null = null;
+let thumbnailPollInterval: ReturnType<typeof setInterval> | null = null;
 
 function startPolling() {
 	if (pollInterval) return;
@@ -29,6 +41,18 @@ function stopPolling() {
 	}
 }
 
+function startThumbnailPolling() {
+	if (thumbnailPollInterval) return;
+	thumbnailPollInterval = setInterval(fetchThumbnailStatus, 2000);
+}
+
+function stopThumbnailPolling() {
+	if (thumbnailPollInterval) {
+		clearInterval(thumbnailPollInterval);
+		thumbnailPollInterval = null;
+	}
+}
+
 async function fetchImportStatus() {
 	const res = await fetch('/api/admin/import-status');
 	if (res.ok) {
@@ -51,11 +75,37 @@ async function triggerImport() {
 	}
 }
 
+async function fetchThumbnailStatus() {
+	const res = await fetch('/api/admin/thumbnail-status');
+	if (res.ok) {
+		thumbnailStatus = await res.json();
+		if (thumbnailStatus!.state === 'RUNNING') {
+			startThumbnailPolling();
+		} else {
+			stopThumbnailPolling();
+		}
+	}
+}
+
+async function triggerThumbnails() {
+	const res = await fetch('/api/admin/generate-thumbnails', { method: 'POST' });
+	if (res.ok) {
+		thumbnailStatus = await res.json();
+		if (thumbnailStatus!.state === 'RUNNING') {
+			startThumbnailPolling();
+		}
+	}
+}
+
 $effect(() => {
 	fetchImportStatus();
+	fetchThumbnailStatus();
 });
 
-onDestroy(() => stopPolling());
+onDestroy(() => {
+	stopPolling();
+	stopThumbnailPolling();
+});
 
 async function backfillVersions() {
 	backfillLoading = true;
@@ -168,5 +218,62 @@ async function backfillFileHashes() {
 				</button>
 			{/if}
 		</div>
+
+		<!-- Thumbnail backfill -->
+		<div class="rounded-sm border border-line bg-surface p-6 shadow-sm">
+			<h2 class="mb-1 font-sans text-sm font-bold text-ink">
+				{m.admin_system_thumbnails_heading()}
+			</h2>
+			<p class="mb-4 text-sm text-ink-2">{m.admin_system_thumbnails_description()}</p>
+
+			{#if thumbnailStatus?.state === 'RUNNING'}
+				<p class="text-sm text-ink-2">
+					{m.admin_system_thumbnails_status_running()}
+					{#if thumbnailStatus.total > 0}
+						<span class="text-ink-3">
+							({thumbnailStatus.processed + thumbnailStatus.skipped + thumbnailStatus.failed} /
+							{thumbnailStatus.total})
+						</span>
+					{/if}
+				</p>
+			{:else if thumbnailStatus?.state === 'DONE'}
+				<p
+					data-testid="thumbnails-status-done"
+					class="mb-4 rounded-sm border border-green-200 bg-green-50 p-3 text-sm text-green-700"
+				>
+					{m.admin_system_thumbnails_status_done({
+						processed: thumbnailStatus.processed,
+						skipped: thumbnailStatus.skipped,
+						failed: thumbnailStatus.failed
+					})}
+				</p>
+				<button
+					data-thumbnails-trigger
+					onclick={triggerThumbnails}
+					class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-80"
+				>
+					{m.admin_system_thumbnails_btn_retry()}
+				</button>
+			{:else if thumbnailStatus?.state === 'FAILED'}
+				<p class="mb-4 rounded-sm border border-red-200 bg-red-50 p-3 text-sm text-red-700">
+					{m.admin_system_thumbnails_status_failed({ message: thumbnailStatus.message })}
+				</p>
+				<button
+					data-thumbnails-trigger
+					onclick={triggerThumbnails}
+					class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-80"
+				>
+					{m.admin_system_thumbnails_btn_retry()}
+				</button>
+			{:else}
+				<button
+					data-thumbnails-trigger
+					onclick={triggerThumbnails}
+					class="rounded-sm bg-primary px-5 py-2 font-sans text-xs font-bold tracking-widest text-primary-fg uppercase transition-opacity hover:opacity-80"
+				>
+					{m.admin_system_thumbnails_btn_start()}
+				</button>
+			{/if}
+		</div>
 	</div>
 </div>