fix(ocr): resume polling on page reload + track single-doc job status

Single-document OCR now creates an OcrJobDocument row so
GET /api/documents/{id}/ocr-status can find running jobs.
OcrAsyncRunner updates the job document status (RUNNING → DONE/FAILED).

Frontend checks OCR status when entering transcription mode —
if a job is running, resumes polling and shows the spinner.

Refs #226

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

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

@@ -251,12 +251,28 @@ function handleParagraphClick(annotationId: string) {
 	);
 }
 
-// Load blocks when transcribe mode is entered and set default panel mode
+async function checkOcrStatus() {
+	if (!doc?.id) return;
+	try {
+		const res = await fetch(`/api/documents/${doc.id}/ocr-status`);
+		if (!res.ok) return;
+		const status = await res.json();
+		if ((status.status === 'PENDING' || status.status === 'RUNNING') && status.jobId) {
+			ocrRunning = true;
+			pollOcrJob(status.jobId);
+		}
+	} catch {
+		// best-effort
+	}
+}
+
+// Load blocks and check OCR status when transcribe mode is entered
 $effect(() => {
 	if (transcribeMode) {
 		loadTranscriptionBlocks().then(() => {
 			panelMode = transcriptionBlocks.length > 0 ? 'read' : 'edit';
 		});
+		checkOcrStatus();
 	}
 });