feat(frontend): add typed API contract layer

Made-with: Cursor

frontend/src/api/index.ts

@@ -1 +1,3 @@
-export {};
+export type { CreateRacePayload, Race, RacesQuery, RaceStatus, UpdateRacePayload } from "./types";
+export { ApiError, getApiErrorMessage } from "./errors";
+export { getRaceById, getRaces, createRace, updateRace, deleteRace } from "./races";