test: explicit tag delete-cascade cleanup test (MPS-24)

Operator-facing concern: 'can I experiment with auto-suggested tags
and undo cleanly without DB cruft?' Yes — Phase 1 already wires the
SQLAlchemy cascade via ProductTag.tag's backref
(cascade='all, delete-orphan'), but the existing functional test only
verified the Tag row went away. This adds an end-to-end test that:

1. Applies a tag to 3 products via action=apply_suggestion (creating
   3 ProductTag rows).
2. Deletes the tag via action=delete.
3. Asserts: Tag row gone, all 3 ProductTag rows gone, products survive
   with empty .tags.

Proves reversibility for an operator testing categorizations on the
suggest-then-approve loop.

make_post_sell/tests/test_functional.py

@@ -8618,3 +8618,63 @@ class TestHomeLayoutAndTags(_AuthenticatedBase):
         if res.status_int == 302:
             res = res.follow()
         self.assertIn("Could not apply suggestion", res.body.decode())
+
+    def test_tag_delete_cascade_cleans_product_associations(self):
+        """Deleting a tag cascade-deletes every ProductTag row pointing
+        at it. The operator can apply experimental tags, dislike them,
+        and delete them — no orphan rows accumulate in the DB."""
+        shop, products = self._make_shop_with_products(
+            "cascade-shop",
+            [
+                ("Alpha Product", "Body one."),
+                ("Beta Product", "Body two."),
+                ("Gamma Product", "Body three."),
+            ],
+        )
+        product_ids = [str(p.id) for p in products]
+
+        # Apply a tag to all three via the suggest-apply path
+        self.testapp.post(
+            f"/s/{shop.id}/tags",
+            {
+                "action": "apply_suggestion",
+                "label": "Experimental",
+                "product_ids": ",".join(product_ids),
+            },
+        )
+        from ..models.tag import get_tag_by_shop_and_slug
+        from ..models.product_tag import ProductTag
+        self.dbsession.expire_all()
+        tag = get_tag_by_shop_and_slug(self.dbsession, shop, "experimental")
+        self.assertIsNotNone(tag)
+        tag_id = tag.id
+        self.assertEqual(
+            self.dbsession.query(ProductTag)
+            .filter(ProductTag.tag_id == tag_id)
+            .count(),
+            3,
+        )
+
+        # Delete the tag — cascade must clean all ProductTag rows
+        self.testapp.post(
+            f"/s/{shop.id}/tags",
+            {"action": "delete", "tag_slug": "experimental"},
+        )
+        self.dbsession.expire_all()
+        # Tag itself gone
+        self.assertIsNone(
+            get_tag_by_shop_and_slug(self.dbsession, shop, "experimental")
+        )
+        # Zero orphan ProductTag rows for the deleted tag
+        self.assertEqual(
+            self.dbsession.query(ProductTag)
+            .filter(ProductTag.tag_id == tag_id)
+            .count(),
+            0,
+        )
+        # Products themselves survive + carry no tags
+        from ..models.product import get_product_by_id
+        for pid in product_ids:
+            p = get_product_by_id(self.dbsession, pid)
+            self.assertIsNotNone(p)
+            self.assertEqual(list(p.tags), [])