feat(relationship): add INVALID_RELATIONSHIP_DATES error code

New 400 error code for a relationship whose toDate precedes its fromDate,
registered in all four sites at once (ErrorCode.java, errors.ts,
getErrorMessage, messages/{de,en,es}.json) per constitution §3.6. Thrown by
the relationship date validation introduced in the following commit.

Refs #837
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -122,6 +122,8 @@ public enum ErrorCode {
     CIRCULAR_RELATIONSHIP,
     /** A relationship with the same (person, relatedPerson, type) already exists. 409 */
     DUPLICATE_RELATIONSHIP,
+    /** A relationship's toDate is before its fromDate. 400 */
+    INVALID_RELATIONSHIP_DATES,
 
     // --- Geschichten (Stories) ---
     /** A Geschichte (story) with the given ID does not exist, or is a DRAFT and the caller lacks BLOG_WRITE. 404 */