feat(products): export hosted detail URL for fetched images; snapshot carries status (SD-0002 §6.5.5, INV-16)

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

backend/tests/test_products_serialize.py

@@ -104,6 +104,24 @@ def test_more_images_than_variants_emits_image_only_rows():
     assert [r["Image Src"] for r in rows] == ["https://x/a.jpg", "https://x/b.jpg", "https://x/c.jpg"]
 
 
+def test_fetched_image_serializes_hosted_detail_url():
+    product = CatalogProduct(
+        id=1, handle="lamp", title="Lamp", option_names=(None, None, None),
+        fields={"title": "Lamp", "status": "active"},
+        variants=[CatalogVariant(id=1, options=(None, None, None), position=1, fields={})],
+        images=[
+            CatalogImage(id=55, source_url="https://m.example/a.png", position=1,
+                         alt_text=None, status="fetched"),
+            CatalogImage(id=56, source_url="https://m.example/b.png", position=2,
+                         alt_text=None, status="failed"),
+        ],
+    )
+    rows = list(serialize._product_rows(product, base_url="https://shop.test"))
+    srcs = [r.get("Image Src") for r in rows if r.get("Image Src")]
+    assert "https://shop.test/api/products/images/55/detail" in srcs  # fetched -> hosted
+    assert "https://m.example/b.png" in srcs                          # failed -> source kept
+
+
 import random
 
 from app.domains.products import codec, diff, validate