feat(api,db,ui): forecast champion selector slice C — forecast decision, business summary & promotion (#362)

[w7-mgfcode]

Summary

Slice C of the Forecast Champion Selector — Forecast Decision, Business Summary & Promotion. Closes the A→B→C series (tracking issue #362). Additive to the model_selection slice; extends the existing /visualize/champion page (no new route/nav).

Backend

• decision.py (new, pure) — z_for_service_level (fixed table, no scipy), compute_forecast_decision (King safety-stock z·σ·√L), forecast_peak_low, BIAS_EXPLANATION.
• schemas.py — TrainSelectedRequest, ForecastDecisionParams, ForecastDecision, PromoteRequest, PromoteResponse; extended ForecastSummary/TrainWinnerResponse/PredictWinnerResponse.
• models.py — 7 additive columns on ModelSelectionRun (trained_model_type, is_override, override_reason, champion_run_id, promoted_alias, promotion_decision, feature_frame_version).
• service.py — train_selected (override, validated against candidate models), promote (lazy RegistryService: create_run→RUNNING→register artifact→SUCCESS→create_alias + audit), predict_winner returns the decision tuple; feature_frame_version persisted at run-creation.
• routes.py — POST /train-selected, POST /promote, optional decision body on /predict.
• Migration e4f5a6b7c8d9 — 7 additive cols; up/down round-trip verified.

Frontend

• Slice C types + extended forecast summary in types/api.ts.
• useTrainWinner/useTrainSelected/usePredictWinner/usePromoteChampion hooks.
• New components/champion-selector/decision/ — DecisionSection orchestration root (keyed by selectionId) + 8 presentational panels, each with tests.
• champion.tsx renders <DecisionSection> below Slice B results when terminal + winner.

Docs

• New docs/user-guide/champion-selector-guide.md + cross-link from feature-reference.md.

Validation

• ruff + format clean; mypy/pyright clean (except documented lightgbm/xgboost optional-dep noise — CI installs the extras).
• Backend 1839 unit + 17 model_selection integration green.
• Frontend tsc/lint clean + 319 vitest green.
• Migration up/down clean. Full dogfood passed (Level 1–4).

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• Commit unit tests in branch feat/champion-selector-slice-c

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