fix: resolve Starlette cookie deprecation warnings

Set cookies on client instance instead of passing per-request to avoid
the deprecation warning about ambiguous cookie persistence behavior.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_user_defaults_integration.py

@@ -94,6 +94,7 @@ class TestFeedUserDefaults:
     ):
         """Successful feed-given saves user defaults."""
         csrf_token = "test-csrf-token"
+        client.cookies.set("csrf_token", csrf_token)
         response = client.post(
             "/actions/feed-given",
             data={
@@ -105,7 +106,6 @@ class TestFeedUserDefaults:
                 "X-Oidc-Username": "ppetru",
                 **make_csrf_headers(csrf_token),
             },
-            cookies={"csrf_token": csrf_token},
         )
         assert response.status_code == 200
 
@@ -148,6 +148,7 @@ class TestFeedUserDefaults:
     ):
         """Unknown users are rejected by auth middleware."""
         csrf_token = "test-csrf-token"
+        client.cookies.set("csrf_token", csrf_token)
         response = client.post(
             "/actions/feed-given",
             data={
@@ -159,7 +160,6 @@ class TestFeedUserDefaults:
                 "X-Oidc-Username": "unknown_user",
                 **make_csrf_headers(csrf_token),
             },
-            cookies={"csrf_token": csrf_token},
         )
         # Unknown user is rejected by auth middleware
         assert response.status_code == 401