feat(api): standardize REST contracts and auth

tests/api/test_model_import_endpoints.py

@@ -43,7 +43,7 @@ def test_system_admin_can_import_model_without_project_membership(monkeypatch):
     )
 
     response = client.post(
-        f"/api/v1/admin/projects/{project_id}/model/import",
+        f"/api/v1/admin/projects/{project_id}/model-imports",
         files={"file": ("desktop-model.inp", VALID_INP)},
     )
 
@@ -61,7 +61,7 @@ def test_non_admin_is_denied_model_import():
     client = TestClient(app)
 
     response = client.post(
-        f"/api/v1/admin/projects/{uuid4()}/model/import",
+        f"/api/v1/admin/projects/{uuid4()}/model-imports",
         files={"file": ("desktop-model.inp", VALID_INP)},
     )