wickworks v0.2.0 — per-indicator preflight + structured 400

Each requested indicator's min-bar requirement is now checked before
any compute work runs. Under-fed requests are rejected with a 400 that
lists every deficit at once — no silent all-null series, no whack-a-mole.

Breaking: insufficient bars is now 400 (structured body) instead of 422
(plain string). 422 stays for Pydantic-level malformed payloads.