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/db.ts

@@ -1,4 +1,6 @@
+import type { SQL } from "drizzle-orm";
 import { drizzle } from "drizzle-orm/mysql2";
+import type { ResultSetHeader } from "mysql2";
 import mysql from "mysql2/promise";
 import * as relations from "@/db/relations";
 import * as schema from "@/db/schema";
@@ -62,6 +64,26 @@ export const db = globalForDb.db ?? createDb();
 
 if (env.NODE_ENV !== "production") globalForDb.db = db;
 
+export function rowsFrom<T>(result: unknown): T[] {
+	const [first] = result as [unknown, unknown];
+	return (Array.isArray(first) ? first : []) as T[];
+}
+
+export async function queryRows<T>(query: SQL | string): Promise<T[]> {
+	return rowsFrom<T>(await db.execute(query));
+}
+
+export async function execResult(
+	query: SQL | string,
+): Promise<ResultSetHeader> {
+	const [first] = (await db.execute(query)) as unknown as [unknown, unknown];
+	return first as ResultSetHeader;
+}
+
+export async function affectedRows(query: SQL | string): Promise<number> {
+	return (await execResult(query)).affectedRows;
+}
+
 export type Db = ReturnType<typeof createDb>;
 
 export * from "@/db/schema";