fix(study): use date-based comparison with 3 AM boundary for due cards

Instead of comparing due timestamps exactly (card.due <= now), compare
against the next 3 AM boundary so all cards due within the current
study day appear at once. This prevents new cards from trickling in
throughout the day when FSRS fuzz spreads due times.

https://claude.ai/code/session_01FeDztLcyGofd6nxh8ct7a3

6 files changed, 94 insertions, 9 deletions

diff --git a/src/client/db/repositories.test.ts b/src/client/db/repositories.test.ts
index 448cb9e..38cf3fb 100644
--- a/src/client/db/repositories.test.ts
+++ b/src/client/db/repositories.test.ts
@@ -299,7 +299,8 @@ describe("localCardRepository", () => {
 	describe("findDueCards", () => {
 		it("should return cards that are due", async () => {
 			const pastDue = new Date(Date.now() - 60000);
-			const future = new Date(Date.now() + 60000);
+			// Use a date far enough in the future to be beyond the next 3 AM boundary
+			const future = new Date(Date.now() + 2 * 24 * 60 * 60 * 1000);
 
 			const card1 = await localCardRepository.create({
 				deckId,
diff --git a/src/client/db/repositories.ts b/src/client/db/repositories.ts
index e01254e..5121bae 100644
--- a/src/client/db/repositories.ts
+++ b/src/client/db/repositories.ts
@@ -1,4 +1,5 @@
 import { v4 as uuidv4 } from "uuid";
+import { getEndOfStudyDayBoundary } from "../../shared/date";
 import {
 	CardState,
 	db,
@@ -140,11 +141,11 @@ export const localCardRepository = {
 	 * Get due cards for a deck
 	 */
 	async findDueCards(deckId: string, limit?: number): Promise<LocalCard[]> {
-		const now = new Date();
+		const boundary = getEndOfStudyDayBoundary();
 		const query = db.cards
 			.where("deckId")
 			.equals(deckId)
-			.filter((card) => card.deletedAt === null && card.due <= now);
+			.filter((card) => card.deletedAt === null && card.due < boundary);
 
 		const cards = await query.toArray();
 		// Sort by due date ascending
diff --git a/src/client/pages/DeckDetailPage.tsx b/src/client/pages/DeckDetailPage.tsx
index be4dc90..97f8378 100644
--- a/src/client/pages/DeckDetailPage.tsx
+++ b/src/client/pages/DeckDetailPage.tsx
@@ -7,6 +7,7 @@ import { FontAwesomeIcon } from "@fortawesome/react-fontawesome";
 import { useAtomValue } from "jotai";
 import { Suspense } from "react";
 import { Link, useParams } from "wouter";
+import { getEndOfStudyDayBoundary } from "../../shared/date";
 import { cardsByDeckAtomFamily, deckByIdAtomFamily } from "../atoms";
 import { ErrorBoundary } from "../components/ErrorBoundary";
 import { LoadingSpinner } from "../components/LoadingSpinner";
@@ -27,9 +28,9 @@ function DeckHeader({ deckId }: { deckId: string }) {
 function DeckStats({ deckId }: { deckId: string }) {
 	const { data: cards } = useAtomValue(cardsByDeckAtomFamily(deckId));
 
-	// Count cards due today
-	const now = new Date();
-	const dueCards = cards.filter((card) => new Date(card.due) <= now);
+	// Count cards due today (study day boundary is 3:00 AM)
+	const boundary = getEndOfStudyDayBoundary();
+	const dueCards = cards.filter((card) => new Date(card.due) < boundary);
 
 	return (
 		<div className="bg-white rounded-xl border border-border/50 p-6 mb-6">
diff --git a/src/server/repositories/card.ts b/src/server/repositories/card.ts
index ac03bc6..2f14149 100644
--- a/src/server/repositories/card.ts
+++ b/src/server/repositories/card.ts
@@ -1,4 +1,5 @@
-import { and, eq, isNull, lte, sql } from "drizzle-orm";
+import { and, eq, isNull, lt, sql } from "drizzle-orm";
+import { getEndOfStudyDayBoundary } from "../../shared/date.js";
 import { db } from "../db/index.js";
 import {
 	CardState,
@@ -189,6 +190,7 @@ export const cardRepository: CardRepository = {
 		now: Date,
 		limit: number,
 	): Promise<Card[]> {
+		const boundary = getEndOfStudyDayBoundary(now);
 		const result = await db
 			.select()
 			.from(cards)
@@ -196,7 +198,7 @@ export const cardRepository: CardRepository = {
 				and(
 					eq(cards.deckId, deckId),
 					isNull(cards.deletedAt),
-					lte(cards.due, now),
+					lt(cards.due, boundary),
 				),
 			)
 			.orderBy(cards.due)
@@ -205,6 +207,7 @@ export const cardRepository: CardRepository = {
 	},
 
 	async countDueCards(deckId: string, now: Date): Promise<number> {
+		const boundary = getEndOfStudyDayBoundary(now);
 		const result = await db
 			.select({ count: sql<number>`count(*)::int` })
 			.from(cards)
@@ -212,7 +215,7 @@ export const cardRepository: CardRepository = {
 				and(
 					eq(cards.deckId, deckId),
 					isNull(cards.deletedAt),
-					lte(cards.due, now),
+					lt(cards.due, boundary),
 				),
 			);
 		return result[0]?.count ?? 0;
diff --git a/src/shared/date.test.ts b/src/shared/date.test.ts
new file mode 100644
index 0000000..1c61a15
--- /dev/null
+++ b/src/shared/date.test.ts
@@ -0,0 +1,58 @@
+import { describe, expect, it } from "vitest";
+import { getEndOfStudyDayBoundary } from "./date";
+
+describe("getEndOfStudyDayBoundary", () => {
+	it("should return next day 3:00 AM when current time is after 3:00 AM", () => {
+		// Feb 2, 2026 10:00 AM
+		const now = new Date(2026, 1, 2, 10, 0, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getFullYear()).toBe(2026);
+		expect(boundary.getMonth()).toBe(1);
+		expect(boundary.getDate()).toBe(3);
+		expect(boundary.getHours()).toBe(3);
+		expect(boundary.getMinutes()).toBe(0);
+		expect(boundary.getSeconds()).toBe(0);
+	});
+
+	it("should return today 3:00 AM when current time is before 3:00 AM", () => {
+		// Feb 2, 2026 1:30 AM
+		const now = new Date(2026, 1, 2, 1, 30, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getFullYear()).toBe(2026);
+		expect(boundary.getMonth()).toBe(1);
+		expect(boundary.getDate()).toBe(2);
+		expect(boundary.getHours()).toBe(3);
+		expect(boundary.getMinutes()).toBe(0);
+		expect(boundary.getSeconds()).toBe(0);
+	});
+
+	it("should return next day 3:00 AM when current time is exactly 3:00 AM", () => {
+		// Feb 2, 2026 3:00 AM
+		const now = new Date(2026, 1, 2, 3, 0, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getDate()).toBe(3);
+		expect(boundary.getHours()).toBe(3);
+	});
+
+	it("should return next day 3:00 AM when current time is 11:59 PM", () => {
+		// Feb 2, 2026 11:59 PM
+		const now = new Date(2026, 1, 2, 23, 59, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getDate()).toBe(3);
+		expect(boundary.getHours()).toBe(3);
+	});
+
+	it("should handle month boundaries correctly", () => {
+		// Jan 31, 2026 15:00
+		const now = new Date(2026, 0, 31, 15, 0, 0, 0);
+		const boundary = getEndOfStudyDayBoundary(now);
+
+		expect(boundary.getMonth()).toBe(1); // February
+		expect(boundary.getDate()).toBe(1);
+		expect(boundary.getHours()).toBe(3);
+	});
+});
diff --git a/src/shared/date.ts b/src/shared/date.ts
new file mode 100644
index 0000000..583d2a6
--- /dev/null
+++ b/src/shared/date.ts
@@ -0,0 +1,21 @@
+/**
+ * Returns the end-of-day boundary for due card comparison.
+ *
+ * The "study day" is defined as 3:00 AM to the next day's 3:00 AM.
+ * All cards with `due < boundary` are considered due for the current study day.
+ *
+ * - If current time >= 3:00 AM, boundary = tomorrow 3:00 AM local time
+ * - If current time < 3:00 AM, boundary = today 3:00 AM local time
+ */
+export function getEndOfStudyDayBoundary(now: Date = new Date()): Date {
+	const boundary = new Date(now);
+	boundary.setMinutes(0, 0, 0);
+
+	if (boundary.getHours() >= 3) {
+		// Move to next day
+		boundary.setDate(boundary.getDate() + 1);
+	}
+
+	boundary.setHours(3);
+	return boundary;
+}