[REVIEW-001] Products: Customer reviews & star ratings

## Overview
Allow customers who have purchased a product to leave a star rating and written review. Reviews are displayed on the Product Detail Page and influence the product's overall rating shown in the catalog.

## Acceptance Criteria
- [ ] Review form on PDP (#4): star rating (1–5), title, body (min 20 chars), optional photo upload (placeholder)
- [ ] `POST /api/reviews` — submit a review (requires auth + verified purchase)
- [ ] `GET /api/reviews/:productId` — returns paginated reviews sorted by helpfulness
- [ ] Aggregate rating (average + count) shown on product cards in the catalog
- [ ] Verified Purchase badge on reviews tied to an actual order
- [ ] Helpful / Not Helpful voting on each review
- [ ] Flag review for moderation
- [ ] Review moderation queue (admin only) — approve/reject before public display
- [ ] Sort options: Most Recent, Most Helpful, Highest Rating, Lowest Rating

## Depends On
- #5 AUTH-001 — reviewer must be authenticated
- #14 ORD-001 — verified purchase status requires order records

## Blocks
- #23 PERF-001 (review data adds weight — lazy loading needed)