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/app/api/admin/studio/inspect/route.test.ts

@@ -13,7 +13,10 @@ vi.mock("@/lib/api-handler", () => ({
 		return handler;
 	},
 }));
-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("@/lib/services/furni-data", () => ({ readFurniData: mocks.read }));
 vi.mock("@/lib/services/furni-asset-dirs", () => ({
 	getFurniAssetDirs: async () => ({