Convert remaining small / api-only features to React Query

Part of #1946 — Redux → React Query migration (Stage 1).

Goal: convert the remaining small, self-contained features to React Query — a catch-all for conversions not owned by another sub-issue.

Scope note: the features currently here use no Redux, so converting them removes no reducer — it is an optional React Query caching/dedupe improvement, not de-Redux work. Keep this issue scoped to whatever isn't covered elsewhere; prune as items move.

Current contents

• courseware/course/celebration (single POST)
• alerts/enrollment-alert (api-only)
• preferences-unsubscribe/data (query + mutation)
• instructor-toolbar/masquerade-widget/data (query + mutation; its data/ is already just api.ts)

Explicitly not here

• recommendations — done in the infrastructure / pattern-setter issue.
• bookmark — moves to the courseware/data issue (its bookmarked state lives in the shared courseware units model read by UnitTitleSlot / UnitButton, so it can't be cut cleanly until then).

Verify: each converted feature works against mocked queries; no new Redux introduced.