feat(26-01): implement discovery service with cursor pagination

- getPopularSetups: public setups ordered by item count desc, composite cursor pagination
- getRecentGlobalItems: global items ordered by createdAt desc, ISO timestamp cursor
- getTrendingCategories: category counts ordered desc, null categories excluded, simple limit
- Shared CursorPage<T> response shape with hasMore and nextCursor fields

src/server/services/discovery.service.ts

@@ -0,0 +1,130 @@
+import { and, count, desc, eq, isNotNull, lt, sql } from "drizzle-orm";
+import { db as prodDb } from "../../db/index.ts";
+import { globalItems, setups, setupItems, users } from "../../db/schema.ts";
+
+type Db = typeof prodDb;
+
+interface CursorPage<T> {
+	items: T[];
+	nextCursor: string | null;
+	hasMore: boolean;
+}
+
+/**
+ * Get popular public setups ordered by item count descending.
+ * Cursor format: "{itemCount}_{id}" for stable composite pagination.
+ * Only public setups (isPublic=true) are returned.
+ */
+export async function getPopularSetups(
+	db: Db = prodDb,
+	limit = 6,
+	cursor?: string,
+): Promise<
+	CursorPage<{
+		id: number;
+		name: string;
+		createdAt: Date;
+		itemCount: number;
+		creatorName: string | null;
+	}>
+> {
+	// Fetch more rows when cursor provided to ensure we can filter and still fill the page
+	const fetchLimit = cursor ? limit * 2 + limit + 1 : limit + 1;
+
+	const rows = await db
+		.select({
+			id: setups.id,
+			name: setups.name,
+			createdAt: setups.createdAt,
+			itemCount: sql<number>`CAST(COUNT(${setupItems.id}) AS INT)`,
+			creatorName: users.displayName,
+		})
+		.from(setups)
+		.leftJoin(setupItems, eq(setupItems.setupId, setups.id))
+		.leftJoin(users, eq(users.id, setups.userId))
+		.where(eq(setups.isPublic, true))
+		.groupBy(setups.id, setups.name, setups.createdAt, users.displayName)
+		.orderBy(
+			desc(sql<number>`COUNT(${setupItems.id})`),
+			desc(setups.id),
+		)
+		.limit(fetchLimit);
+
+	// Apply cursor filter in JS (composite cursor: itemCount_id)
+	let filtered = rows;
+	if (cursor) {
+		const parts = cursor.split("_");
+		const cursorCount = Number(parts[0]);
+		const cursorId = Number(parts[1]);
+		filtered = rows.filter(
+			(r) =>
+				r.itemCount < cursorCount ||
+				(r.itemCount === cursorCount && r.id < cursorId),
+		);
+	}
+
+	const hasMore = filtered.length > limit;
+	const items = hasMore ? filtered.slice(0, limit) : filtered;
+	const nextCursor =
+		hasMore && items.length > 0
+			? `${items[items.length - 1].itemCount}_${items[items.length - 1].id}`
+			: null;
+
+	return { items, nextCursor, hasMore };
+}
+
+/**
+ * Get recently added global catalog items ordered by createdAt descending.
+ * Cursor is an ISO timestamp string — returns items created before that time.
+ */
+export async function getRecentGlobalItems(
+	db: Db = prodDb,
+	limit = 8,
+	cursor?: string,
+): Promise<CursorPage<typeof globalItems.$inferSelect>> {
+	const conditions = cursor
+		? [lt(globalItems.createdAt, new Date(cursor))]
+		: [];
+
+	const rows = await db
+		.select()
+		.from(globalItems)
+		.where(conditions.length ? and(...conditions) : undefined)
+		.orderBy(desc(globalItems.createdAt))
+		.limit(limit + 1);
+
+	const hasMore = rows.length > limit;
+	const items = hasMore ? rows.slice(0, limit) : rows;
+	const nextCursor =
+		hasMore && items.length > 0
+			? items[items.length - 1].createdAt.toISOString()
+			: null;
+
+	return { items, nextCursor, hasMore };
+}
+
+/**
+ * Get trending categories ordered by global item count descending.
+ * Excludes items where category IS NULL.
+ * Simple limit pagination — no cursor (categories are a bounded list).
+ */
+export async function getTrendingCategories(
+	db: Db = prodDb,
+	limit = 12,
+): Promise<Array<{ name: string; itemCount: number }>> {
+	const rows = await db
+		.select({
+			name: globalItems.category,
+			itemCount: count(globalItems.id),
+		})
+		.from(globalItems)
+		.where(isNotNull(globalItems.category))
+		.groupBy(globalItems.category)
+		.orderBy(desc(count(globalItems.id)))
+		.limit(limit);
+
+	return rows.map((r) => ({
+		name: r.name as string,
+		itemCount: r.itemCount,
+	}));
+}