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/services/furniture-id-reservation.test.ts

@@ -3,7 +3,10 @@ import { MySqlDialect } from "drizzle-orm/mysql-core";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const mocks = vi.hoisted(() => ({ execute: vi.fn(), conflict: vi.fn() }));
-vi.mock("@/lib/db", () => ({ db: { execute: mocks.execute } }));
+vi.mock("@/lib/db", async () => ({
+	...(await import("@/test/db-helpers")).createDbHelpers(mocks.execute),
+	db: { execute: mocks.execute },
+}));
 vi.mock("./furni-data", () => ({ findFurniDataIdConflict: mocks.conflict }));
 
 import { reserveFurnitureId } from "./furniture-id-reservation";