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/actions/voucher.test.ts

@@ -6,6 +6,8 @@ vi.mock("next/server", () => ({
 	},
 }));
 
+import { fakeForm } from "@/test/fake-form";
+
 vi.mock("next-auth", () => ({
 	default: vi.fn(() => ({
 		handlers: {},
@@ -82,11 +84,6 @@ vi.mock("next/cache", () => ({
 import { db } from "@/lib/db";
 import { redeem } from "./voucher";
 
-const fakeForm = (data: Record<string, string>) =>
-	({
-		get: (key: string) => data[key] ?? null,
-	}) as unknown as FormData;
-
 beforeEach(() => {
 	vi.clearAllMocks();
 	mockAuth.mockResolvedValue({ user: { id: "1" } });