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/lib/auth/login-core.ts

@@ -1,7 +1,7 @@
 import { sql } from "drizzle-orm";
 import { checkLogin } from "@/lib/auth/password";
 import { cachedQuery, invalidateKey } from "@/lib/cached-db";
-import { db } from "@/lib/db";
+import { queryRows } from "@/lib/db";
 import { siteSettings } from "@/lib/services/site-settings";
 
 export interface LoginUser {
@@ -45,7 +45,7 @@ export async function getLoginUser(
 	return cachedQuery<LoginUser | null>(
 		`login:user:${username}`,
 		async () => {
-			const [result] = await db.execute<{
+			const rows = await queryRows<{
 				id: number;
 				username: string;
 				password: string | null;
@@ -63,16 +63,6 @@ export async function getLoginUser(
 				WHERE username = ${username}
 				LIMIT 1
 			`);
-			const rows = result as unknown as Array<{
-				id: number;
-				username: string;
-				password: string | null;
-				rank: number;
-				mail: string | null;
-				mail_verified: string | null;
-				two_factor_confirmed_at: string | null;
-				two_factor_secret: string | null;
-			}>;
 			return rows.length > 0
 				? {
 						id: rows[0].id,