backend-zod-validationlisted

## Overview Add schema validation middleware so every endpoint rejects malformed input with a 400. ## Steps 1. Install `@hono/zod-validator`. 2. Define a Zod schema for each POST and PUT body and attach the validator middleware. 3. Return a structured error payload listing the failing fields. ## Verify Run the API tests, then post an invalid body and confirm a 400 with a field level error list.