fix: add fallback from product.domain when candidates empty

cfc-backend/src/main/java/com/etotem/cfc/service/ProductRecommendationService.java

@@ -135,6 +135,39 @@ public class ProductRecommendationService {
             }
         }
 
+        // 3.5 fallback: 当 dimension_mapping 表无数据时，从 Product.domain 匹配
+        if (candidates.isEmpty()) {
+            LambdaQueryWrapper<Product> domainQw = new LambdaQueryWrapper<Product>()
+                    .eq(Product::getStatus, "on_shelf")
+                    .gt(Product::getStock, 0)
+                    .orderByAsc(Product::getSortOrder);
+            if (dimensionCode != null && !dimensionCode.isEmpty()) {
+                domainQw.eq(Product::getDomain, dimensionCode);
+            }
+            if (!purchasedIds.isEmpty()) {
+                domainQw.notIn(Product::getId, purchasedIds);
+            }
+            SortUtil.applySort(domainQw);
+            List<Product> domainMatched = productMapper.selectList(domainQw);
+            for (Product p : domainMatched) {
+                Map<String, Object> item = new LinkedHashMap<>();
+                item.put("id", p.getId());
+                item.put("productId", p.getId());
+                item.put("name", p.getName());
+                item.put("coverImage", p.getCoverImage());
+                item.put("price", p.getPrice());
+                item.put("memberPrice", p.getPrice());
+                item.put("productType", p.getProductType());
+                item.put("source", "dimension");
+                item.put("url", "/pages/discover/product-detail/product-detail?id=" + p.getId());
+                String reason = buildRecommendationReason(p, dimensionCode, memberScores);
+                item.put("reason", reason);
+                double matchScore = calculateMatchScore(p, dimensionCode, memberScores);
+                item.put("matchScore", matchScore);
+                candidates.add(item);
+            }
+        }
+
         // 4. 关联关系推荐（动态：画像缺口、共购、复购）
         List<Map<String, Object>> assocResults = queryAssociationRecommendations(
                 dimensionCode, familyId, memberId, purchasedIds, excludeProductIds, limit - candidates.size());