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/java/org/raddatz/familienarchiv/model/Document.java

@@ -43,6 +43,13 @@ public class Document {
     @Column(name = "file_hash", length = 64)
     private String fileHash;
 
+    // S3 key of the generated thumbnail (e.g. "thumbnails/{docId}.jpg"); null until generated
+    @Column(name = "thumbnail_key")
+    private String thumbnailKey;
+
+    @Column(name = "thumbnail_generated_at")
+    private LocalDateTime thumbnailGeneratedAt;
+
     // Originaler Dateiname beim Upload (z.B. "Brief_Oma_1940.pdf")
     @Column(name = "original_filename", nullable = false)
     @Schema(requiredMode = Schema.RequiredMode.REQUIRED)

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;