feat(backend): diff-vs-previous-run reporting for product-scan accuracy harness

Ports the DO-harness's auto-diff-vs-previous-run reporting into
accuracy-check-scan.mts: prints a per-field, per-split (Training/
Validation) delta against the last product_accuracy_history.jsonl entry
and calls out regressions/improvements explicitly, plus classifier
method distribution and average confidence as informational context.

Also adds the real held-out validation photo set into
sources/product-test-images/ (75 photos, one per current SKU class) with
its README documenting the drop-photo -> label -> re-run workflow, so the
harness's Validation Set split actually has images to score against.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xsxk4ZkDQVVaLUcixDcqb5

backend/docs/scan-product.md

@@ -136,6 +136,32 @@ get mangled. Verify in `docker logs`: "DINOv2 index loaded with N reference
 images", "Using classifier weights: <new dated file>". Full worked example:
 `plans/next-enhancements.md` task 2.1.
 
+## Accuracy regression harness
+
+`backend/scripts/accuracy-check-scan.mts` — mirrors the DO-flow's
+`pfm-web-app/scripts/accuracy-check.mts`. Hits the live `/api/scan-pfm` for
+every labeled image in `sources/product_manual_labels.json`, checks 3 fields
+(`no_sku`, `nama_item`, `expiry_date`) against ground truth, and splits into:
+- **Training Set** — gallery photos under `foto-kemasan-v2/` (the classifier's
+  own reference images; scores here measure memorization, not generalization).
+- **Validation Set** — flat filenames dropped into
+  `sources/product-test-images/` (a real held-out set; see that folder's
+  `README.md` for the drop-photo → label → re-run workflow via
+  `/manual-label-scan`).
+
+Every run appends to `sources/product_accuracy_history.jsonl` and **auto-diffs
+against the previous run**: the printed summary shows a Δ column per field per
+split, flags field/image-level regressions and improvements, and reports
+classifier method (`dinov2_similarity`/`yolo_classifier`) distribution +
+average confidence as informational context (not scored pass/fail, since
+DINOv2's "confidence" is a raw cosine similarity, not a calibrated
+probability — see Stage 1 above). This is what makes it safe to tune
+`classify_ocr_server.py` and immediately see whether a change helped or hurt.
+
+```bash
+node scripts/accuracy-check-scan.mts   # from backend/
+```
+
 ## Operational notes
 
 - **Env vars**: `CLASSIFIER_SERVER_URL`, `PIPELINE_URL` (gateway, set in compose);
@@ -156,20 +182,18 @@ images", "Using classifier weights: <new dated file>". Full worked example:
 ## Known gaps & future recommendations
 
 Tracked ones (see `plans/next-enhancements.md`):
-- **§6.1–6.3 ground truth**: today's "Save Ground Truth" stores the model's own
-  predictions (only the SKU is editable) and uploads get phantom
-  `uploaded-<timestamp>.jpg` keys with no image persisted; §6 plans the editable
-  annotation page, persisted uploads, and a scan accuracy harness.
 - **Dataset thinness**: 2–16 photos/class caps both classifiers; every new real
-  photo (especially non-studio, in-warehouse shots) matters. The §6.3 harness
-  should report gallery vs. uploaded-photo accuracy separately — gallery photos
-  are training data, so scores on them measure memorization.
+  photo (especially non-studio, in-warehouse shots) matters. The harness above
+  already reports gallery (training) vs. held-out (validation) accuracy
+  separately — but as of this writing `sources/product-test-images/` is empty,
+  so the Validation Set is still 0 images and every published number so far is
+  a training/memorization score. Dropping real photos there is the next step,
+  not yet done.
 
 Additional recommendations (not yet tasks — promote via `e`/`n` when wanted):
-1. **Use `extracted_sku` in match ranking.** An exact 8-digit SKU hit read off
-   the label is far stronger evidence than fuzzy name similarity, yet ranking
-   currently ignores it. Suggested: exact `no_sku` match pins rank 1; blend name
-   similarity for the rest.
+1. ~~Use `extracted_sku` in match ranking.~~ **Done** — `product-scan.ts`'s
+   `classifyAndMatchProduct` already pins rank 1 to an exact `no_sku` match
+   (score forced to 1.0) before falling back to name similarity.
 2. **Fuse DINOv2 and YOLO instead of primary/fallback** (e.g. agreement boosts
    confidence; disagreement flags for review) — cheap, both already load.
 3. **"Not a known product" handling**: DINOv2 always returns *some* class; add a