Add Pinterest tag/filter for shop products

- New is_pinterest boolean on ShopProductListingModel, settable via the
  existing create/update product endpoints (multipart form field)
- /shop-products/discover accepts ?pinterest=true to show only
  Pinterest-tagged products across shops; skips the tagged-posts merge
  under this filter since those posts aren't Pinterest-tagged themselves

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

models/ShopProductListingModel.js

@@ -88,6 +88,11 @@ const shopProductListingSchema = new mongoose.Schema({
     enum: ['pending_shop_approval', 'active', 'inactive', 'rejected'],
     default: 'pending_shop_approval',
     index: true
+  },
+  is_pinterest: {
+    type: Boolean,
+    default: false,
+    index: true
   }
 })