feat(POS-008): completed feature

work/artifacts/POS-008/architect.md

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+# POS-008 — Architect
+
+## Feature
+POS Phase 1 ticket 008: POST /pos/sales idempotent transaction.
+
+## Objetivo
+POST /pos/sales: creates a POS sale atomically (order + line items + payments + cash session update) in a DB transaction. Idempotent via idempotency key.
+
+## Diseño
+- `CreatePosSaleUseCase` takes: idempotencyKey, cashSessionId, terminalId, userId, items[], payments[]
+- Step 1: idempotency check (return existing if key found)
+- Step 2: verify cash session OPEN (FOR UPDATE lock)
+- Step 3: compute totals
+- Step 4: INSERT order
+- Step 5: INSERT line items
+- Step 6: INSERT payments + update cash session expected_cash_cents
+- All in BEGIN/COMMIT/ROLLBACK transaction
+- Provider types: pos_cash, pos_card, pos_other (not regular payment providers)