[OPS-009] Operations: Returns portal — initiate & track returns

## Overview
Build the self-service returns portal so customers can initiate and track product returns without contacting support.

## Acceptance Criteria
- [ ] Accessible via "Return Items" on any delivered order in order history (#17)
- [ ] Multi-step return flow: select items → select reason → choose refund method → confirm
- [ ] Return reasons: defective, wrong item, changed mind, arrived too late, other
- [ ] `POST /api/returns` — creates a return request with a unique RMA (Return Merchandise Authorization) number
- [ ] `GET /api/returns/:rmaId` — tracks return status
- [ ] Return statuses: `requested` → `label_sent` → `received` → `refund_processed`
- [ ] Email sent when return is approved (via #19 notification service)
- [ ] Refund methods: original payment method (stubbed), store credit
- [ ] Return window: 30 days from delivery date (enforced server-side)

## ⚠️ Downstream Impact
If #14 ORD-001 or #17 ACC-012 slip, this ticket is also blocked. That creates a 3-ticket delay chain in Sprint 4.

## Depends On
- #14 ORD-001 — return references the original order
- #17 ACC-012 — return is initiated from order history

## Blocks
- #22 SUP-002 (support tickets often reference RMA numbers)