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/test/db-helpers.ts

@@ -0,0 +1,23 @@
+import type { ResultSetHeader } from "mysql2";
+
+export function rowsFrom<T>(result: unknown): T[] {
+	const [first] = result as [unknown, unknown];
+	return (Array.isArray(first) ? first : []) as T[];
+}
+
+export function createDbHelpers<Query>(execute: (query: Query) => unknown) {
+	async function queryRows<T>(query: Query): Promise<T[]> {
+		return rowsFrom<T>(await execute(query));
+	}
+
+	async function execResult(query: Query): Promise<ResultSetHeader> {
+		const [first] = (await execute(query)) as [unknown, unknown];
+		return first as ResultSetHeader;
+	}
+
+	async function affectedRows(query: Query): Promise<number> {
+		return (await execResult(query)).affectedRows;
+	}
+
+	return { rowsFrom, queryRows, execResult, affectedRows };
+}

src/test/fake-form.ts

@@ -0,0 +1,7 @@
+export function fakeForm(
+	data: Record<string, string | null | undefined>,
+): FormData {
+	return {
+		get: (key: string) => data[key] ?? null,
+	} as unknown as FormData;
+}