feat: Add review_helpful_votes table (OQI-002)

Created table to track user votes on course reviews:
- education.review_helpful_votes
- Constraint: one vote per user per review
- Indexes on user_id and review_id

This table supports the "mark as helpful" feature in course reviews.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ddl/schemas/education/tables/16-review_helpful_votes.sql

@@ -0,0 +1,26 @@
+-- =====================================================
+-- TABLE: education.review_helpful_votes
+-- =====================================================
+-- Proyecto: OrbiQuant IA (Trading Platform)
+-- Módulo: OQI-002 - Education
+-- Especificación: Votos de "útil" en reviews de cursos
+-- =====================================================
+
+CREATE TABLE education.review_helpful_votes (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+
+  user_id UUID NOT NULL REFERENCES auth.users(id) ON DELETE CASCADE,
+  review_id UUID NOT NULL REFERENCES education.course_reviews(id) ON DELETE CASCADE,
+
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+
+  CONSTRAINT unique_user_review_vote UNIQUE(user_id, review_id)
+);
+
+-- Índices
+CREATE INDEX idx_review_helpful_votes_user ON education.review_helpful_votes(user_id);
+CREATE INDEX idx_review_helpful_votes_review ON education.review_helpful_votes(review_id);
+
+-- Comentarios
+COMMENT ON TABLE education.review_helpful_votes IS 'Votos de usuarios que encontraron útil una review';
+COMMENT ON CONSTRAINT unique_user_review_vote ON education.review_helpful_votes IS 'Un usuario solo puede votar una vez por review';