feat(#248): add TAG_NOT_FOUND, TAG_MERGE_SELF, TAG_MERGE_INVALID_TARGET error codes

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

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

@@ -79,10 +79,16 @@ public enum ErrorCode {
     TRAINING_ALREADY_RUNNING,
 
     // --- Tags ---
+    /** A tag with the given ID does not exist. 404 */
+    TAG_NOT_FOUND,
     /** The supplied color token is not in the allowed palette. 400 */
     INVALID_TAG_COLOR,
     /** Setting this parent would create a cycle in the tag hierarchy. 400 */
     TAG_CYCLE_DETECTED,
+    /** Merge source and target are the same tag. 400 */
+    TAG_MERGE_SELF,
+    /** The merge target is a descendant of the source tag. 400 */
+    TAG_MERGE_INVALID_TARGET,
 
     // --- Generic ---
     /** Request validation failed (missing or malformed fields). 400 */