Add flashcards feature with spaced repetition scheduling

Adds a full flashcards system: FSRS-based scheduling, cloze/basic
card types, CSV import/export, media storage, and study UI, wired
into navigation and the dashboard alongside existing question flows.

api/QuestionsApi/Services/Flashcards/FlashcardScheduling.cs

@@ -0,0 +1,14 @@
+namespace QuestionsApi.Services.Flashcards;
+
+// Small pure helpers pulled out of FlashcardStudyService/FlashcardScheduleStore so the
+// daily-limit and sibling-bury rules can be unit tested without a live Mongo instance.
+public static class FlashcardScheduling
+{
+    // A card reviewed on a given local study day buries its siblings until the next local day.
+    public static DateTime NextStudyDayStart(DateTime studyDayStartUtc) => studyDayStartUtc.Date.AddDays(1);
+
+    // Effective per-day budget: deck override wins over the global default, minus what's
+    // already been consumed on the client's current local study day.
+    public static int ResolveDailyBudget(int globalLimit, int? deckOverride, int consumedToday) =>
+        Math.Max(0, (deckOverride ?? globalLimit) - consumedToday);
+}