Build a small `check_jwt(token, now)` helper that ties 4 module-2 ideas together: 1. **Format** — must have exactly 3 dot-separated parts; else return None. 2. **Decode payload** — middle segment is base64url-encoded JSON. 3. **Expiry** — payload `exp` (unix seconds) must be > `now`; else None. 4. **Subject** — return `payload["sub"]` (the user_id). The `now` parameter is injected so tests stay deterministic — a fake-clock pattern you'll reuse in module 3. The harness prints results for: valid token / expired / malformed (only 2 parts) / missing exp.