feat(annotations): add ANNOTATION_UPDATE_FAILED error code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -49,6 +49,8 @@ public enum ErrorCode {
     // --- Annotations ---
     /** The annotation with the given ID does not exist. 404 */
     ANNOTATION_NOT_FOUND,
+    /** The annotation position/size could not be saved. 500 */
+    ANNOTATION_UPDATE_FAILED,
 
     // --- Transcription Blocks ---
     /** The transcription block with the given ID does not exist. 404 */