F-189 — Implementer evidence

Delivered

Migration 056_pos_return_lines.js:
- Adds orders_items.returned_quantity integer NOT NULL DEFAULT 0 with CHECK.
- Extends inventory_movements.operation CHECK to allow 'return'.
- Creates pos_returns ledger with UNIQUE (order_id, idempotency_key) for replay.
New ApplyPosReturnUseCase:
- Locks order and stock rows.
- Validates per-line return bounds; rejects over-return with POS_RETURN_EXCEEDS_AVAILABLE.
- Requires full return for free items with POS_RETURN_FREE_ITEM_PARTIAL.
- Restores inventory, decrements cash session expected cash proportionally.
- Emits reporting_payment_lines row with status='refund' / 'partial_refund'.
- Inserts orders_order_history, pos_returns, audit events.
- Idempotent via pos_returns.idempotency_key.
New buildPosReturnReceipt returning an R-<original> numbered receipt with negative totals and originalReceiptNumber.
New POST /pos/sales/:id/returns endpoint (POS roles + x-terminal-id binding).
New GET /pos/sales/:id/items endpoint returning order items with returnedQuantity.
Removed legacy POST /pos/sales/:id/refund.
POS cashier UI: new ReturnModal exposed via a Devolver artículos button on the receipt modal; shows per-line steppers, reason and refund total; on success replaces the active receipt with the return receipt.
Type updates so both POS, admin and backend see originalReceiptNumber / isReturn flags.

Backend, POS, admin typecheck: PASS.
Real PostgreSQL sequential suite: 365/365 PASS (5 new in pos-returns.itest.ts).
Migration cycle (fresh up / second no-op / full down / re-up): 4/4 PASS.
Targeted ESLint + Prettier on every changed file: PASS.
./scripts/verify.sh: PASS.

Backend global lint remains unchanged; no new errors.
Boundary check stays at one pre-existing security import violation; F-189 introduces none.
Untracked upload JPGs excluded from the feature commit.