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>

backend/src/test/java/org/raddatz/familienarchiv/service/OcrServiceTest.java

@@ -8,6 +8,7 @@ import org.mockito.junit.jupiter.MockitoExtension;
 import org.raddatz.familienarchiv.exception.DomainException;
 import org.raddatz.familienarchiv.exception.ErrorCode;
 import org.raddatz.familienarchiv.model.*;
+import org.raddatz.familienarchiv.repository.OcrJobDocumentRepository;
 import org.raddatz.familienarchiv.repository.OcrJobRepository;
 
 import java.util.UUID;
@@ -24,6 +25,7 @@ class OcrServiceTest {
     @Mock OcrHealthClient ocrHealthClient;
     @Mock DocumentService documentService;
     @Mock OcrJobRepository ocrJobRepository;
+    @Mock OcrJobDocumentRepository ocrJobDocumentRepository;
     @Mock OcrAsyncRunner ocrAsyncRunner;
 
     @InjectMocks OcrService ocrService;