feat(document): reject end date without RANGE precision (#678)

Add the second validateDateRange predicate mirroring
chk_meta_date_end_only_for_range, so a direct API client that sets an end
date without RANGE precision gets a clean 400 INVALID_DATE_RANGE instead of
a 500 (AC6). Shares the code with the end-before-start branch.

Also fix updateDocument_preservesStoredPrecision_whenDtoOmitsIt: its stored
fixture (MONTH + end date) is a state the DB CHECK forbids, so the
carried-over-state guard correctly rejects it. Switched to RANGE + end —
the only DB-valid non-null-end combo — preserving the test's intent.

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

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

@@ -486,6 +486,12 @@ public class DocumentService {
             throw DomainException.badRequest(ErrorCode.INVALID_DATE_RANGE,
                     "meta_date_end must not be before meta_date");
         }
+        // Mirrors chk_meta_date_end_only_for_range. API-only: the edit form clears the
+        // end field off-RANGE, so this branch closes the same 500 class for direct clients.
+        if (doc.getMetaDateEnd() != null && doc.getMetaDatePrecision() != DatePrecision.RANGE) {
+            throw DomainException.badRequest(ErrorCode.INVALID_DATE_RANGE,
+                    "meta_date_end is only allowed when meta_date_precision is RANGE");
+        }
     }
 
     @Transactional