test(document): lock in accepted RANGE cases — equal/after/open/null-start (#678)

Cover AC2 (end == start), AC3 (open-ended, end null) and AC4 (null start +
end set, which must not reject or NPE), plus end-after-start. Guards the
guard against future over-rejection that would diverge from the DB CHECK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -256,6 +256,63 @@ class DocumentServiceTest {
         verify(documentRepository, never()).save(any());
     }
 
+    @Test
+    void updateDocument_acceptsRange_whenEndEqualsStart() throws Exception {
+        // AC2: the DB CHECK is end >= start, so equal dates are valid.
+        UUID id = UUID.randomUUID();
+        Document doc = docForRangeUpdate(id);
+        when(documentRepository.findById(id)).thenReturn(Optional.of(doc));
+        when(documentRepository.save(any())).thenReturn(doc);
+
+        LocalDate same = LocalDate.of(1917, 1, 10);
+        documentService.updateDocument(id, rangeDto(same, same), null, null);
+
+        assertThat(doc.getMetaDateEnd()).isEqualTo(same);
+        verify(documentRepository, atLeastOnce()).save(any());
+    }
+
+    @Test
+    void updateDocument_acceptsRange_whenEndAfterStart() throws Exception {
+        UUID id = UUID.randomUUID();
+        Document doc = docForRangeUpdate(id);
+        when(documentRepository.findById(id)).thenReturn(Optional.of(doc));
+        when(documentRepository.save(any())).thenReturn(doc);
+
+        documentService.updateDocument(id,
+                rangeDto(LocalDate.of(1917, 1, 10), LocalDate.of(1917, 1, 11)), null, null);
+
+        verify(documentRepository, atLeastOnce()).save(any());
+    }
+
+    @Test
+    void updateDocument_acceptsRange_whenEndIsNull_openEnded() throws Exception {
+        // AC3: an open-ended range (no end) is valid.
+        UUID id = UUID.randomUUID();
+        Document doc = docForRangeUpdate(id);
+        when(documentRepository.findById(id)).thenReturn(Optional.of(doc));
+        when(documentRepository.save(any())).thenReturn(doc);
+
+        documentService.updateDocument(id,
+                rangeDto(LocalDate.of(1917, 1, 10), null), null, null);
+
+        verify(documentRepository, atLeastOnce()).save(any());
+    }
+
+    @Test
+    void updateDocument_acceptsRange_whenStartNullAndEndSet() throws Exception {
+        // AC4: mirrors the DB "meta_date IS NULL" escape — must NOT reject (and must not NPE).
+        UUID id = UUID.randomUUID();
+        Document doc = docForRangeUpdate(id);
+        when(documentRepository.findById(id)).thenReturn(Optional.of(doc));
+        when(documentRepository.save(any())).thenReturn(doc);
+
+        documentService.updateDocument(id,
+                rangeDto(null, LocalDate.of(1917, 1, 11)), null, null);
+
+        assertThat(doc.getMetaDateEnd()).isEqualTo(LocalDate.of(1917, 1, 11));
+        verify(documentRepository, atLeastOnce()).save(any());
+    }
+
     // ─── deleteTagCascading ───────────────────────────────────────────────────
 
     @Test