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/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;
+}