feat(product): implement sprint 9 product engagement module

docs/backend-platform/business-apis.md

@@ -49,6 +49,54 @@ Tenant rule:
 Must not change:
 - item identifiers/price semantics relied on frontend checkout/cart logic.
 
+## /products/{id}/rating
+Purpose:
+- return product rating aggregate for engagement UI.
+
+High-level response shape:
+- average rating
+- total reviews
+- star distribution (5..1)
+
+Tenant rule:
+- aggregate must be computed only from tenant-visible reviews.
+
+## /products/{id}/reviews
+Purpose:
+- paginated review feed for product page.
+
+High-level response shape:
+- review list
+- page/pageSize/total metadata
+
+Tenant rule:
+- only tenant-allowed and moderation-approved reviews.
+
+## /products/{id}/questions
+Purpose:
+- paginated questions and answers feed for product page.
+
+High-level response shape:
+- question list with answers
+- page/pageSize/total metadata
+
+Tenant rule:
+- only tenant-visible questions/answers.
+
+## /products/{id}/reviews (POST)
+Purpose:
+- create review (rating/title/text/anonymous).
+
+Must not change:
+- request validation semantics expected by frontend engagement form.
+
+## /products/{id}/questions (POST)
+Purpose:
+- create product question (text/anonymous).
+
+Must not change:
+- acknowledgement contract expected by frontend engagement form.
+
 ## /categories
 Purpose:
 - category tree retrieval