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

@@ -378,9 +378,7 @@ public class DocumentService {
         // 1. Einfache Felder Update
         doc.setTitle(dto.getTitle());
         doc.setDocumentDate(dto.getDocumentDate());
-        doc.setMetaDatePrecision(dto.getMetaDatePrecision());
+        applyDatePrecision(doc, dto);
-        doc.setMetaDateEnd(dto.getMetaDateEnd());
-        doc.setMetaDateRaw(dto.getMetaDateRaw());
         doc.setLocation(dto.getLocation());
         doc.setTranscription(dto.getTranscription());
         doc.setSummary(dto.getSummary());
@@ -449,6 +447,25 @@ public class DocumentService {
         return saved;
     }
 
+    /**
+     * Applies the three date-precision fields only when the DTO carries them.
+     * A null field means "not submitted" — overwriting the stored value with null
+     * would fabricate a precision the user never chose, the exact dishonesty #666
+     * exists to prevent. A row with a genuinely-unknown precision must keep it when
+     * an unrelated edit (e.g. a location typo) is saved.
+     */
+    private void applyDatePrecision(Document doc, DocumentUpdateDTO dto) {
+        if (dto.getMetaDatePrecision() != null) {
+            doc.setMetaDatePrecision(dto.getMetaDatePrecision());
+        }
+        if (dto.getMetaDateEnd() != null) {
+            doc.setMetaDateEnd(dto.getMetaDateEnd());
+        }
+        if (dto.getMetaDateRaw() != null) {
+            doc.setMetaDateRaw(dto.getMetaDateRaw());
+        }
+    }
+
     @Transactional
     public Document updateDocumentTags(UUID docId, List<String> tagNames) {
         Document doc = documentRepository.findById(docId)

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