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/feature-list.md

@@ -71,7 +71,8 @@ workflow and have no task numbers; see `git log` for real dates/history.
 
 - **6.1** Built standalone annotation page `manual-label-scan/page.tsx` for ground truth editing. Includes image browser, editable fields (`no_sku`, `nama_item`, `expiry_date`, `notes`), and a "Scan with AI" fill-blanks feature — shipped 2026-07-08.
 - **6.2** API + storage groundwork for scan annotation. Extended `api/manual-label-scan` with `GET` list mode and `DELETE`. Persisted uploaded scan photos as base64 images into `sources/product-test-images/`. Made the `scan-pfm` quick-save honest by allowing manual correction before save — shipped 2026-07-08.
-- **6.3** Built `backend/pfm-web-app/scripts/accuracy-check-scan.mts` mirroring the DO-harness architecture, measuring overall match rate plus per-field breakdown (`no_sku`, `expiry_date`) against the new stable labels — shipped 2026-07-08.
+- **6.3** Built `backend/scripts/accuracy-check-scan.mts` mirroring the DO-harness architecture, measuring overall match rate plus per-field breakdown (`no_sku`, `expiry_date`) against the new stable labels — shipped 2026-07-08.
+- **6.4** Ported the DO-harness's auto-diff-vs-previous-run reporting into `accuracy-check-scan.mts`: every run now prints a Δ column per field per split (Training/Validation) vs the last `product_accuracy_history.jsonl` entry, and calls out field- and image-level regressions/improvements explicitly. Added classifier method (`dinov2_similarity`/`yolo_classifier`) distribution and average confidence as informational (non-scoring) context. Created the previously-missing `sources/product-test-images/README.md` documenting the validation-photo drop workflow — shipped 2026-07-13, user-directed `n` request to make algorithm tuning self-verifying.
 
 ### Master Data Management
 - **8.1 & 8.3 CRUD APIs and Web UI**: Created `/api/v1/master/stores` and `/api/v1/master/skus` endpoints alongside a Next.js Admin page (`/admin/master-data`) to visually manage the core reference data used by the OCR matching engine — shipped 2026-07-08.