Feature: Weighted products & scale-generated barcodes (electronic scale, stage 1) #365

Feature: Weighted products & scale-generated barcodes (electronic scale, stage 1)

Summary

Support deli/grocery-style weighted items. Stage 1 = scale-generated labels: many scales print barcodes that embed a PLU + weight/price. Stage 2 (separate, later) = direct HID/USB/serial scale reading. This issue covers stage 1.

Proposed configuration

Prefix         21
Product digits 5
Weight digits  5
Unit           grams

Scan 2101234012507 → product 01234 → 1.250 kg → add quantity 1.25.

Weighted product metadata

products: sale_unit (each|kg|g|lb), allow_fractional_quantity, weight_precision

What already exists

• Order creation already accepts positive numerical (non-integer) quantities (main/routes/orders.ts: quantity <= 0 || !Number.isFinite(quantity) is the only guard) — fractional order math is compatible.
• Concrete inconsistency to fix: held orders require Number.isSafeInteger(item.quantity) (main/routes/held-orders.ts:45), so weighted quantities fail there.
• Barcode lookup is exact and uniqueness-enforced server-side (main/routes/products.ts ~750/851); scanner hook is global/reusable.

Acceptance criteria

• Weighted product metadata fields + migration; fractional quantity works end to end.
• Scale-barcode parser (prefix/PLU/weight config) adds the weighted item with correct quantity/price.
• Held orders accept fractional quantities for weighted products.
• Stage 2 (zero/tare/stable reading from a real scale) tracked separately later.