docs(plan): per-sale expiry tracking design — batch registry + candidate matching

Grilled 2026-07-16 with the user; full decision record in
docs/expiry-tracking-plan.md. Core reframe: expiry is captured once per
batch at DO intake (staff-typed on the stock-entry confirmation page, from
the physical packs), so the cashier scan only MATCHES OCR fragments against
the 1-3 known in-stock batch dates instead of free-reading damaged
dot-matrix prints (proven model-capability ceiling, 2026-07-15). Fallback:
auto-FEFO + 'inferred' flag, zero cashier interaction. No cloud, ever.

- docs/expiry-tracking-plan.md: architecture, matching algorithm spec
  (resolveExpiryFromEvidence), schema/API deltas, phases 1-3, testing plan
- backend plans §13 (13.1-13.4): matcher util + offline tuning, route
  wiring + expiry_source provenance, multi-frame union, dot-matrix
  recognizer fine-tune
- root plans §10 (10.1-10.3): cashier fast path, inferred badge +
  end-of-day review, burst capture for mounted camera
- stock-feature-plan.md: extension note (batch dropdown becomes the
  manual-override path)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q8TumxFDnyVnfsR3mxPXfX

backend/config/scratch_test_ocr.py

@@ -1,20 +0,0 @@
-from PIL import Image
-import numpy as np
-from paddleocr import PaddleOCR
-
-ocr = PaddleOCR(use_textline_orientation=True, lang='en')
-image = Image.open('/app/config/test_img.jpeg').convert('RGB')
-img_arr = np.array(image)
-res_list = list(ocr.predict(img_arr))
-
-texts = res_list[0].get('rec_texts', [])
-dt_polys = res_list[0].get('dt_polys', [])
-
-for idx, (text, poly) in enumerate(zip(texts, dt_polys)):
-    if 'BB05032027' in text or 'BB' in text:
-        print(f"Match: {text}")
-        print("Raw poly:")
-        print(poly)
-        print("Pts computed:")
-        pts = [(float(p[0]), float(p[1])) for p in poly]
-        print(pts)