feat(documents): add DocumentService.getDensity (#385)

Maps the repository's Object[] rows into a DocumentDensityResult and pairs
them with the archive-wide min/max meta_date range. Read-only, no
@Transactional needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -125,6 +125,19 @@ public class DocumentService {
         return titles;
     }
 
+    /**
+     * Per-month document counts for the timeline density widget (issue #385).
+     * Returns only months that have at least one document — the frontend fills
+     * gaps using the {@code minDate}/{@code maxDate} bounds.
+     */
+    public DocumentDensityResult getDensity(LocalDate from, LocalDate to) {
+        List<MonthBucket> buckets = documentRepository.findDensityByMonth(from, to).stream()
+                .map(row -> new MonthBucket((String) row[0], ((Number) row[1]).intValue()))
+                .toList();
+        DocumentDateRangeProjection range = documentRepository.findMinMaxDocumentDate();
+        return new DocumentDensityResult(buckets, range.getMinDate(), range.getMaxDate());
+    }
+
     /**
      * Lädt eine Datei hoch.
      * - Prüft, ob ein Eintrag (aus Excel) schon existiert.