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/admin-vouchers.ts

@@ -1,7 +1,6 @@
 "use server";
 
 import { eq } from "drizzle-orm";
-import type { ResultSetHeader } from "mysql2";
 import { revalidatePath } from "next/cache";
 import { requirePermission } from "@/lib/admin/guard";
 import { positiveBigInt } from "@/lib/api";
@@ -47,7 +46,7 @@ export async function createVoucher(input: {
 	const now = new Date();
 
 	try {
-		const [result] = (await db.insert(WebsiteShopVouchers).values({
+		const [result] = await db.insert(WebsiteShopVouchers).values({
 			code,
 			amount: Math.floor(amount),
 			maxUses,
@@ -55,7 +54,7 @@ export async function createVoucher(input: {
 			expiresAt,
 			createdAt: now,
 			updatedAt: now,
-		})) as unknown as [ResultSetHeader];
+		});
 		revalidatePath("/admin/vouchers");
 		return actionOk({ id: String(result.insertId) });
 	} catch (error) {