fix(dates): preserve stored date precision when edit omits it

updateDocument unconditionally set metaDatePrecision/End/Raw from the DTO,
so saving an unrelated edit (a multipart PUT where the form omits the
precision controls) clobbered the stored precision with null — fabricating
a precision the user never chose. Apply each field only when the DTO carries
it, mirroring the existing metadataComplete/scriptType guards.

Refs #666
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -164,6 +164,33 @@ class DocumentServiceTest {
         assertThat(doc.getMetaDateRaw()).isEqualTo("10.–11. Januar 1917");
     }
 
+    @Test
+    void updateDocument_preservesStoredPrecision_whenDtoOmitsIt() throws Exception {
+        // Editing a doc (e.g. fixing a location typo) without touching the precision
+        // controls must NOT fabricate a precision. The form omits the three precision
+        // fields → they arrive null on the DTO → the stored values must be preserved.
+        UUID id = UUID.randomUUID();
+        Document doc = Document.builder()
+                .id(id)
+                .metaDatePrecision(DatePrecision.MONTH)
+                .metaDateEnd(LocalDate.of(1916, 6, 30))
+                .metaDateRaw("Juni 1916")
+                .receivers(new HashSet<>())
+                .tags(new HashSet<>())
+                .build();
+        when(documentRepository.findById(id)).thenReturn(Optional.of(doc));
+        when(documentRepository.save(any())).thenReturn(doc);
+
+        DocumentUpdateDTO dto = new DocumentUpdateDTO();
+        dto.setLocation("Berlin"); // unrelated edit; precision fields left null
+
+        documentService.updateDocument(id, dto, null, null);
+
+        assertThat(doc.getMetaDatePrecision()).isEqualTo(DatePrecision.MONTH);
+        assertThat(doc.getMetaDateEnd()).isEqualTo(LocalDate.of(1916, 6, 30));
+        assertThat(doc.getMetaDateRaw()).isEqualTo("Juni 1916");
+    }
+
     // ─── deleteTagCascading ───────────────────────────────────────────────────
 
     @Test