docs: require explicit backend for legacy walks

recognization/docs/DATA.md

@@ -38,6 +38,7 @@ Then run the app and scan. Each scan session becomes its own directory:
     000001.jpg
     index.jsonl       one line per recorded frame:
                       {"frame": 123, "path": "000001.jpg", "at": 1699...,
+                       "identity_backend": "gallery",
                        "summary": {...},
                        "tracks": [{"box": [...], "identity": {...},
                                    "tag_identity": {...}, "tag": {...}}]}
@@ -97,9 +98,15 @@ predictions. For the second visit, run from `recognization` with the same frozen
 SESSION="$HOME/dorayaki-data/storeA-visit2/REPLACE_WITH_SESSION"
 YOLO_OFFLINE=1 .venv/bin/python code/eval_live.py "$SESSION" \
   --stock-dir stock-lopia --labels "$SESSION/labels.json" \
-  --replay --identity gallery --device cpu
+  --replay --device cpu
 ```
 
+A recording written by the current server names `identity_backend` on each
+index line, and the evaluator uses that. `--identity gallery` (or `matcher`)
+is required when the index omits it, including recordings from before that
+field existed, and must match the recorded value when both are present. It
+does not silently score the matcher.
+
 Replace `REPLACE_WITH_SESSION` with the actual session name. The evaluator replays
 recorded JPEGs through `LiveSession`, writes `eval_live.json` in that directory,
 and reports backend, loaded head, weight/catalog fingerprints, decision counts,