fix(ocr): unblock event loop during OCR and show errors in UI

OCR engines are CPU-bound and were blocking Uvicorn's single async
event loop, making /health unresponsive during processing. This caused
new OCR requests to fail silently (health check failure → no DB record
→ UI shows NONE). Wrap engine calls in asyncio.to_thread() to keep the
event loop free. Also surface OCR trigger errors in the frontend
instead of silently resetting the spinner.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -7,6 +7,7 @@ import TranscriptionEditView from '$lib/components/TranscriptionEditView.svelte'
 import TranscriptionReadView from '$lib/components/TranscriptionReadView.svelte';
 import TranscriptionPanelHeader from '$lib/components/TranscriptionPanelHeader.svelte';
 import type { TranscriptionBlockData } from '$lib/types';
+import { getErrorMessage } from '$lib/errors';
 
 let { data } = $props();
 
@@ -129,6 +130,7 @@ async function reviewToggle(blockId: string) {
 
 let ocrRunning = $state(false);
 let ocrProgressMessage = $state('');
+let ocrErrorMessage = $state('');
 let ocrPollTimer = $state<ReturnType<typeof setInterval> | null>(null);
 
 function translateOcrProgress(code: string): string {
@@ -154,6 +156,7 @@ function translateOcrProgress(code: string): string {
 
 async function triggerOcr(scriptType: string) {
 	ocrRunning = true;
+	ocrErrorMessage = '';
 	try {
 		const res = await fetch(`/api/documents/${doc.id}/ocr`, {
 			method: 'POST',
@@ -165,10 +168,14 @@ async function triggerOcr(scriptType: string) {
 			pollOcrJob(data.jobId);
 		} else {
 			ocrRunning = false;
+			const body = await res.json().catch(() => null);
+			const code = (body as { code?: string } | null)?.code;
+			ocrErrorMessage = code ? getErrorMessage(code) : m.ocr_status_error();
 		}
 	} catch (e) {
 		console.error('Failed to trigger OCR:', e);
 		ocrRunning = false;
+		ocrErrorMessage = m.ocr_status_error();
 	}
 }
 
@@ -185,6 +192,9 @@ function pollOcrJob(jobId: string) {
 				ocrPollTimer = null;
 				ocrRunning = false;
 				ocrProgressMessage = '';
+				if (job.status === 'FAILED') {
+					ocrErrorMessage = m.ocr_status_error();
+				}
 				await loadTranscriptionBlocks();
 				annotationReloadKey++;
 				panelMode = transcriptionBlocks.length > 0 ? 'read' : 'edit';
@@ -399,6 +409,11 @@ onMount(() => {
 					onClose={() => (transcribeMode = false)}
 				/>
 				<div class="flex-1 overflow-y-auto">
+					{#if ocrErrorMessage}
+						<div class="mx-4 mt-4 rounded-sm border border-red-200 bg-red-50 px-4 py-3">
+							<p class="text-sm text-red-700">{ocrErrorMessage}</p>
+						</div>
+					{/if}
 					{#if ocrRunning}
 						<div class="flex flex-1 flex-col items-center justify-center px-6 py-12 text-center">
 							<svg