refactor(thumbnail): route document access through DocumentService

The Thumbnail trio (ThumbnailService, ThumbnailBackfillService,
ThumbnailAsyncRunner) all injected DocumentRepository directly. They now go
through three new DocumentService delegations:

- findById(UUID): Optional<Document> — no-throw variant for the runner's
  log-and-skip behaviour on missing documents.
- findForThumbnailBackfill() — wraps the existing
  findByFilePathIsNotNullAndThumbnailKeyIsNull query.
- updateThumbnailMetadata(Document) — wraps save() for the post-thumbnail
  entity update.

DocumentService also gains @Lazy on its existing ThumbnailAsyncRunner field
to break the new DocumentService ↔ ThumbnailAsyncRunner cycle. lombok.config
adds @Lazy to copyableAnnotations so the field annotation reaches the
generated constructor parameter.

Refs #417 (C6.2 violations #2, #3, #4).

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

backend/lombok.config

@@ -0,0 +1 @@
+lombok.copyableAnnotations += org.springframework.context.annotation.Lazy