Helpful vote or "was this helpful?" mechanism is displayed

Helpful-vote mechanisms surface the reviews with the highest signal-to-noise ratio, so shoppers reach useful content faster and conversion climbs. Without votes, long-form detailed reviews sit next to one-line "love it" posts with no way to distinguish quality; community ranking is the cheapest moderation layer that exists. Vote data also feeds the "Most Helpful" sort (WCAG-adjacent discoverability) and trains future review-ranking models.

Add a helpful_votes integer column to the reviews table, expose a POST route at api/reviews/[id]/helpful that increments the counter (rate-limited per user), and render a components/HelpfulVote.tsx button beneath each review displaying the current count:

const handleHelpful = async () => {
  await fetch(`/api/reviews/${reviewId}/helpful`, { method: 'POST' })
  setVotes(v => v + 1)
}

Block duplicate votes from the same user by storing a (user_id, review_id) join row.