feat(document): add server-computed hasTranscription to detail payload (#697)

getDocumentById now populates a transient hasTranscription boolean so the
document detail page can gate the transcription entry control at first
paint (no client store, no full block fetch, no layout shift).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/document/Document.java

@@ -177,6 +177,13 @@ public class Document {
     @Builder.Default
     private Set<TrainingLabel> trainingLabels = new HashSet<>();
 
+    // Not persisted — computed per detail fetch so read-only users can tell at first
+    // paint whether there is a transcription to read (DocumentService.getDocumentById).
+    @Transient
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    @Builder.Default
+    private boolean hasTranscription = false;
+
     // The `?v={thumbnailGeneratedAt}` cache-buster is load-bearing: the thumbnail
     // endpoint sends `Cache-Control: private, max-age=31536000, immutable`
     // (DocumentController.getDocumentThumbnail). `immutable` is only safe because

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java

@@ -943,6 +943,7 @@ public class DocumentService {
         Document doc = documentRepository.findById(id)
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.DOCUMENT_NOT_FOUND, "Document not found: " + id));
         tagService.resolveEffectiveColors(doc.getTags());
+        doc.setHasTranscription(transcriptionBlockQueryService.hasBlocks(id));
         return doc;
     }