feat: an embedding identity backend with a calibrated rejection

The external evaluation showed the matcher cannot say "unknown": on 20 shelves whose
products are absent it claimed 47.8% of facings as recognised, and its raw inlier
score cannot be thresholded (foreign shelves' median 6.0 overlaps the real shelf's
5.2 p10, and the worst offenders score 50-190). So the gallery becomes a selectable
identity backend:

    --identity gallery        (or DORAYAKI_IDENTITY=gallery)

Measured on the same 20 foreign shelves: false accepts fall from 47.8% to 13.8% at
the default threshold and to **5%** at 0.60, with a smooth curve to pick an operating
point from. Detection is unchanged (the gallery only names boxes).

What it does NOT fix, measured on a milk shelf where 14 of 107 facings really are
明治 おいしい牛乳 (labelled by eye): precision is 28-45%, no better than the matcher's
39%, and enrolling the shelf from itself (1-3 views per product) is flat at ~25%
top-1. Off-the-shelf DINOv2 is not discriminative enough for fine-grained SKUs - the
same conclusion the literature reaches, now measured here.

The packaging's own text is a real channel and resolution-bound: 51% recall
single-scale, 60% with the tag reader's multi-scale + red-channel trick, 85% where
the text is large (MEGMILK) and 28% where it is small vertical Japanese.

docs/EXTERNAL-EVAL.md records all of it, including the identity architecture it
implies: reject by default, name from the tag when readable, from the packaging text
when legible, from a fine-tuned embedding otherwise, and hand the rest to the
operator.

175 tests pass.

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

recognization/code/app.py

@@ -276,6 +276,14 @@ def create_app(
                 "warm": True,
             },
             "catalog": {"version": engine.catalog_version(), "sku_count": engine.sku_count()},
+            "identity": {
+                "backend": getattr(engine.settings, "identity_backend", "matcher"),
+                "gallery_views": (
+                    engine.identity_gallery().view_count()
+                    if getattr(engine.settings, "identity_backend", "") == "gallery" and engine.identity_gallery()
+                    else None
+                ),
+            },
             "ocr": {
                 # Without this an operator could not tell a broken OCR from a shelf
                 # with no readable tags.
@@ -987,6 +995,8 @@ def main() -> None:
     parser.add_argument("--classify-workers", type=int, default=None)
     parser.add_argument("--live-method", type=str, default=None, choices=list(CLASSIFY_METHODS))
     parser.add_argument("--live-conf", type=float, default=None)
+    parser.add_argument("--identity", type=str, default=None, choices=["matcher", "gallery"],
+                        help="which mechanism names products (default matcher)")
     parser.add_argument("--live-inflight", type=int, default=None,
                         help="classifications in flight per session (demo walks want a larger budget)")
     parser.add_argument("--live-attempts", type=int, default=None,
@@ -1036,6 +1046,8 @@ def main() -> None:
         server.record_frames = args.record_frames
     if args.record_every is not None:
         server.record_every = args.record_every
+    if args.identity is not None:
+        engine_settings.identity_backend = args.identity
     if args.live_inflight is not None:
         engine_settings.live_inflight = args.live_inflight
     if args.live_attempts is not None: