refactor(db): typed query helpers, shared test FormData helper

Replace raw db.execute tuple casts with queryRows/rowsFrom/execResult/
affectedRows helpers from lib/db, drop redundant mysql2 casts on typed
query builders, and centralize per-test fakeForm into test/fake-form.
Update db mocks in tests so helpers resolve against mocked execute.

src/app/(site)/radio/leaderboard/page.tsx

@@ -3,7 +3,7 @@ import { getTranslations } from "next-intl/server";
 import Link from "@/components/link";
 import { ContentCard, EmptyState, RankBadge } from "@/components/public/ui";
 import { UserAvatarThumbnail } from "@/components/shared/user-avatar-thumbnail";
-import { db } from "@/lib/db";
+import { queryRows } from "@/lib/db";
 
 type LeaderRow = {
 	username: string;
@@ -15,13 +15,13 @@ async function loadRows(): Promise<
 	{ username: string; look: string; points: number }[]
 > {
 	try {
-		const [rows] = (await db.execute(sql`
+		const rows = await queryRows<LeaderRow>(sql`
 			SELECT u.username AS username, u.look AS look, p.points AS points
 			FROM radio_listener_points p
 			INNER JOIN users u ON u.id = p.user_id
 			ORDER BY p.points DESC
 			LIMIT 50
-		`)) as unknown as [LeaderRow[], unknown];
+		`);
 		return (rows ?? []).map((r) => ({
 			username: r.username,
 			look: r.look,