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/Models/FlashcardModels.cs

@@ -10,6 +10,14 @@ public static class FlashcardTypes
     public static readonly HashSet<string> All = [Basic, Cloze];
 }
 
+// A reserved, never-authenticatable owner id. Decks/notes owned by this id are shared,
+// admin-curated content visible to every user (studied via their own independent schedule
+// rows) rather than a real user's private content.
+public static class FlashcardOwners
+{
+    public const string System = "system";
+}
+
 public static class FlashcardRatings
 {
     public const string Again = "again";
@@ -43,6 +51,7 @@ public sealed class FlashcardDeck
     public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
     public DateTime? ArchivedUtc { get; set; }
+    [BsonIgnore] public bool IsSystem => UserId == FlashcardOwners.System;
 }
 
 public sealed class FlashcardImageReference
@@ -69,6 +78,7 @@ public sealed class FlashcardNote
     public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
     public DateTime? ArchivedUtc { get; set; }
+    [BsonIgnore] public bool IsSystem => UserId == FlashcardOwners.System;
 }
 
 public sealed class FlashcardSchedule