feat: two demo shelves, enrolled, plus the price-card filter they forced out

Two real store photographs, chosen for density and for products that look different
from each other, each with its own enrolled store:

  demo-images/demo-image-1-mediya-dairy.jpg   CC0        MEDIYA dairy case
      21 products kept (13 price cards dropped), 16 recognised, all 14 products named
  demo-images/demo-image-2-milk-case.jpg      CC BY 2.0  supermarket milk case
      56 detections, 49 recognised, all 13 products named

Both were enrolled the way a store would be: the products were read off a contact
sheet of the detections, their crops became the store's gallery (stock-mediya,
stock-milk150), and a head was trained on them. `build_store_from_shelf.py` is that
flow.

The filter came out of the first image. Every Japanese supermarket photo has the same
artifact: the detector boxes the shelf-edge price cards as products, because a card is
roughly the shape of a pack - 13 of 34 detections on image 1 were cards. Their text
separates them cleanly: a card's characters are 30-44% digits and it prints the
tax-excluded price (本体), while a product's are 3-19% digits. Cards are now dropped
before classification, and the count is reported as `price_cards_dropped` rather than
hidden.

demo-images/README.md records how to run both, what the demo shows, and the honest
caveat: both shelves are enrolled, so the recognition rate is in-distribution - what
the app does on a shelf it has been onboarded to, not on one it has never seen.

175 tests pass.

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

recognization/stock-milk150/stock.csv

@@ -0,0 +1,14 @@
+SKU_ID,product_name,receipt_date,expiration_date,number_in,number_out,number_on,image_link,price
+sku_k01,明治 おいしい牛乳,2026-09-25,2026-10-15,48,20,28,'',198
+sku_k02,ゆうパック 優良牛乳,2026-09-25,2026-10-15,48,20,28,'',130
+sku_k03,酪農家限定 低温殺菌牛乳,2026-09-25,2026-10-15,48,20,28,'',248
+sku_k04,北海道牛乳,2026-09-25,2026-10-15,48,20,28,'',178
+sku_k05,MEGMILK,2026-09-25,2026-10-15,48,20,28,'',158
+sku_k06,毎日牛乳,2026-09-25,2026-10-15,48,20,28,'',168
+sku_k07,タカナシ 低温殺菌牛乳,2026-09-25,2026-10-15,48,20,28,'',258
+sku_k08,タカナシ 殺菌低脂肪牛乳,2026-09-25,2026-10-15,48,20,28,'',198
+sku_k09,毎日骨太,2026-09-25,2026-10-15,48,20,28,'',148
+sku_k10,カルシウム入り低脂肪乳,2026-09-25,2026-10-15,48,20,28,'',145
+sku_k11,無脂肪牛乳,2026-09-25,2026-10-15,48,20,28,'',138
+sku_k12,低脂肪牛乳,2026-09-25,2026-10-15,48,20,28,'',120
+sku_k13,ミルク,2026-09-25,2026-10-15,48,20,28,'',260