feat(api): add forecast champion selector backend

[w7-mgfcode]

Backend-only model_selection vertical slice per PRPs/forecast-champion-selector-backend.md.

Trains/backtests a set of candidate forecasting models for a (store, product) pair, ranks them, selects a champion, and optionally auto-trains/predicts the winner. Persists a model_selection run and exposes a stable backend contract for a future UI PRP.

Champion-selector UI is a deliberate follow-up PRP (this one is backend-only by design).

[w7-mgfcode]

Implemented in 645047e feat(api,db): forecast champion selector backend (merge 6c3f8d4); shipped in v0.2.21. Closing.