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/banners.ts

@@ -1,7 +1,6 @@
 "use server";
 
 import { eq } from "drizzle-orm";
-import type { ResultSetHeader } from "mysql2";
 import { z } from "zod";
 import { db, WebsiteBanner } from "@/lib/db";
 import { PERMS } from "@/lib/permissions";
@@ -24,9 +23,7 @@ const bannerSchema = z.object({
 export const createBanner = adminAction(
 	{ permission: PERMS.BANNERS_EDIT, schema: bannerSchema },
 	async (ctx) => {
-		const [result] = (await db
-			.insert(WebsiteBanner)
-			.values(ctx.data)) as unknown as [ResultSetHeader];
+		const [result] = await db.insert(WebsiteBanner).values(ctx.data);
 		const id = Number(result.insertId);
 		logAudit({
 			userId: ctx.session.user.id,