System decks/notes are owned by a reserved sentinel user id, reusing
all existing per-user store/index plumbing unchanged. Regular users
see them under a separate "Official Decks" section, can study them
with fully independent per-user FSRS progress (lazily materialized on
first visit), but can never edit or archive them - enforced both in
the client (no Edit/Archive controls) and server-side (ownership guard
on every mutating endpoint). Includes a small idempotent seed command
(`dotnet run -- seed-flashcards`) for testing, since there's no in-app
authoring UI yet.
