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-shop.ts

@@ -1,7 +1,6 @@
 "use server";
 
 import { eq } from "drizzle-orm";
-import type { ResultSetHeader } from "mysql2";
 import { revalidatePath } from "next/cache";
 import { redirect } from "next/navigation";
 import { requirePermission } from "@/lib/admin/guard";
@@ -44,7 +43,7 @@ export async function createShopArticle(formData: FormData): Promise<void> {
 	const now = new Date();
 	const costs = reqUInt(formData, "costs");
 	try {
-		const [result] = (await db.insert(WebsiteShopArticles).values({
+		const [result] = await db.insert(WebsiteShopArticles).values({
 			name,
 			info: String(formData.get("info") ?? "")
 				.normalize("NFC")
@@ -71,7 +70,7 @@ export async function createShopArticle(formData: FormData): Promise<void> {
 			position: reqUInt(formData, "position"),
 			createdAt: now,
 			updatedAt: now,
-		})) as unknown as [ResultSetHeader];
+		});
 		await logStaffActivity({
 			staffId: staff.id,
 			action: "shop_create",