Add system (curated) vs. user-uploaded flashcards

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.

api/QuestionsApi/Services/Flashcards/FlashcardDeckStore.cs

@@ -33,6 +33,21 @@ public sealed class FlashcardDeckStore
     public async Task<FlashcardDeck?> GetAsync(string userId, string deckId) =>
         await _decks.Find(v => v.UserId == userId && v.Id == deckId).FirstOrDefaultAsync();
 
+    // Read-only: the caller's own decks plus every system (shared, admin-curated) deck. Never
+    // used to back a write path - mutations must stay strictly own-user via ListAsync/GetAsync.
+    public Task<List<FlashcardDeck>> ListVisibleAsync(string userId) =>
+        _decks.Find(v => (v.UserId == userId || v.UserId == FlashcardOwners.System) && v.ArchivedUtc == null)
+            .SortBy(v => v.Name).ToListAsync();
+
+    public async Task<FlashcardDeck?> GetVisibleAsync(string userId, string deckId) =>
+        await _decks.Find(v => (v.UserId == userId || v.UserId == FlashcardOwners.System) && v.Id == deckId)
+            .FirstOrDefaultAsync();
+
+    // No owner filter at all - used only by the controller's mutation guard to check IsSystem
+    // regardless of who's asking.
+    public async Task<FlashcardDeck?> GetAnyAsync(string deckId) =>
+        await _decks.Find(v => v.Id == deckId).FirstOrDefaultAsync();
+
     public async Task<FlashcardDeck> GetOrCreateDefaultDeckAsync(string userId)
     {
         var existing = await _decks.Find(v => v.UserId == userId && v.IsDefault).FirstOrDefaultAsync();