feat(document): getThumbnailUrl appends URL-encoded timestamp as cache-buster

Matches the shape the frontend previously built via
encodeURIComponent(thumbnailGeneratedAt), so the backend is now the
single source of truth for the thumbnail URL convention (#309).

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

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

@@ -8,6 +8,8 @@ import org.hibernate.annotations.UpdateTimestamp;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import io.swagger.v3.oas.annotations.media.Schema;
 
+import java.net.URLEncoder;
+import java.nio.charset.StandardCharsets;
 import java.time.LocalDate;
 import java.time.LocalDateTime;
 import java.util.HashSet;
@@ -134,6 +136,8 @@ public class Document {
 
     public String getThumbnailUrl() {
         if (thumbnailKey == null) return null;
-        return "/api/documents/" + id + "/thumbnail";
+        String base = "/api/documents/" + id + "/thumbnail";
+        if (thumbnailGeneratedAt == null) return base;
+        return base + "?v=" + URLEncoder.encode(thumbnailGeneratedAt.toString(), StandardCharsets.UTF_8);
     }
 }

backend/src/test/java/org/raddatz/familienarchiv/model/DocumentTest.java

@@ -2,6 +2,7 @@ package org.raddatz.familienarchiv.model;
 
 import org.junit.jupiter.api.Test;
 
+import java.time.LocalDateTime;
 import java.util.UUID;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -36,4 +37,25 @@ class DocumentTest {
         assertThat(doc.getThumbnailUrl())
                 .isEqualTo("/api/documents/" + id + "/thumbnail");
     }
+
+    @Test
+    void getThumbnailUrl_includesCacheBuster_whenBothKeyAndGeneratedAtPresent() {
+        UUID id = UUID.fromString("aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee");
+        LocalDateTime generatedAt = LocalDateTime.of(2026, 4, 23, 14, 30, 45);
+        Document doc = Document.builder()
+                .id(id)
+                .title("Brief")
+                .originalFilename("brief.pdf")
+                .status(DocumentStatus.UPLOADED)
+                .thumbnailKey("thumbnails/" + id + ".jpg")
+                .thumbnailGeneratedAt(generatedAt)
+                .build();
+
+        // frontend equivalent: `?v=${encodeURIComponent(doc.thumbnailGeneratedAt)}`
+        // where thumbnailGeneratedAt is the ISO-8601 string Jackson serialises.
+        // LocalDateTime.toString() produces "2026-04-23T14:30:45"; encodeURIComponent
+        // turns ":" into "%3A" but leaves "T" and digits alone.
+        String expected = "/api/documents/" + id + "/thumbnail?v=2026-04-23T14%3A30%3A45";
+        assertThat(doc.getThumbnailUrl()).isEqualTo(expected);
+    }
 }