feat(db): add thumbnail_key and thumbnail_generated_at to documents

Adds two nullable columns to the documents table and their JPA mappings
on the Document entity. Both are left out of the OpenAPI required-mode
schema so the generated TypeScript type exposes them as optional.

Refs #307

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

backend/src/main/resources/db/migration/V52__add_document_thumbnails.sql

@@ -0,0 +1,3 @@
+ALTER TABLE documents
+    ADD COLUMN thumbnail_key VARCHAR(255),
+    ADD COLUMN thumbnail_generated_at TIMESTAMP;