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.

backend/src/test/java/org/raddatz/familienarchiv/document/DocumentServiceTest.java

@@ -2321,4 +2321,56 @@ class DocumentServiceTest {
         assertThat(documentService.save(doc)).isEqualTo(doc);
         verify(documentRepository).save(doc);
     }
+
+    // ─── getDensity ────────────────────────────────────────────────────────────
+
+    @Test
+    void getDensity_returnsEmptyResult_whenArchiveIsEmpty() {
+        when(documentRepository.findDensityByMonth(null, null)).thenReturn(List.of());
+        DocumentDateRangeProjection emptyRange = mock(DocumentDateRangeProjection.class);
+        when(emptyRange.getMinDate()).thenReturn(null);
+        when(emptyRange.getMaxDate()).thenReturn(null);
+        when(documentRepository.findMinMaxDocumentDate()).thenReturn(emptyRange);
+
+        DocumentDensityResult result = documentService.getDensity(null, null);
+
+        assertThat(result.buckets()).isEmpty();
+        assertThat(result.minDate()).isNull();
+        assertThat(result.maxDate()).isNull();
+    }
+
+    @Test
+    void getDensity_mapsRepositoryRowsToMonthBuckets() {
+        List<Object[]> rows = List.of(
+                new Object[]{"1915-08", 2L},
+                new Object[]{"1915-09", 1L}
+        );
+        when(documentRepository.findDensityByMonth(null, null)).thenReturn(rows);
+        DocumentDateRangeProjection range = mock(DocumentDateRangeProjection.class);
+        when(range.getMinDate()).thenReturn(LocalDate.of(1915, 8, 3));
+        when(range.getMaxDate()).thenReturn(LocalDate.of(1915, 9, 1));
+        when(documentRepository.findMinMaxDocumentDate()).thenReturn(range);
+
+        DocumentDensityResult result = documentService.getDensity(null, null);
+
+        assertThat(result.buckets()).extracting(MonthBucket::month)
+                .containsExactly("1915-08", "1915-09");
+        assertThat(result.buckets()).extracting(MonthBucket::count)
+                .containsExactly(2, 1);
+        assertThat(result.minDate()).isEqualTo(LocalDate.of(1915, 8, 3));
+        assertThat(result.maxDate()).isEqualTo(LocalDate.of(1915, 9, 1));
+    }
+
+    @Test
+    void getDensity_passesFromAndToBoundsToRepository() {
+        when(documentRepository.findDensityByMonth(any(), any())).thenReturn(List.of());
+        DocumentDateRangeProjection range = mock(DocumentDateRangeProjection.class);
+        when(documentRepository.findMinMaxDocumentDate()).thenReturn(range);
+
+        LocalDate from = LocalDate.of(1914, 1, 1);
+        LocalDate to = LocalDate.of(1918, 12, 31);
+        documentService.getDensity(from, to);
+
+        verify(documentRepository).findDensityByMonth(from, to);
+    }
 }