add inventory sorting to the api

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

trade-hub/backend/tests/test_inventory.py

@@ -189,3 +189,27 @@ def test_catalog_is_not_mutated_by_inventory_operations():
     carrack = client.get(f"/api/v1/items/{CARROCK}").json()
     assert "quantity" not in carrack
     assert "notes" not in carrack
+
+
+def test_sort_by_category_groups_categories():
+    client = authed_client()
+    add(client, CARROCK)
+    add(client, RIFLE)
+    items = client.get("/api/v1/me/inventory?sort=category").json()["items"]
+    assert [i["class_name"] for i in items] == [RIFLE, CARROCK]
+
+
+def test_sort_by_quantity_is_descending():
+    client = authed_client()
+    add(client, CARROCK, quantity=5)
+    add(client, RIFLE, quantity=1)
+    items = client.get("/api/v1/me/inventory?sort=quantity").json()["items"]
+    assert [i["class_name"] for i in items] == [CARROCK, RIFLE]
+
+
+def test_sort_defaults_to_name_and_rejects_unknown_values():
+    client = authed_client()
+    add(client, CARROCK)
+    listing = client.get("/api/v1/me/inventory").json()
+    assert [i["class_name"] for i in listing["items"]] == [CARROCK]
+    assert client.get("/api/v1/me/inventory?sort=bogus").status_code == 422