Forecast Explainability & Driver Attribution slice

[w7-mgfcode]

New app/features/explainability/ vertical slice — rule-based, deterministic forecast explanations for the three baseline forecasters (naive / seasonal_naive / moving_average).

Scope:

• Driver-contribution attribution (each explainer reproduces its forecaster's exact h=1 value) + advisory retail reason codes (stockout / promotion / lifecycle / holiday / insufficient-history), computed time-safely (<= as_of_date)
• forecast_explanation persistence table + Alembic migration + alembic/env.py registration
• 3 endpoints: GET /explain/runs/{run_id}, GET /explain/jobs/{job_id}, POST /explain/forecast
• Frontend explanation panels on the run-detail and forecast pages
• SHAP deliberately out of scope (rule-based MVP; adding it would need its own PRP + ADR)

Plan: PRPs/PRP-28-forecast-explainability-driver-attribution.md

[w7-mgfcode]

Shipped to dev in PR #231 (merge commit af1a5be) — explainability slice with /explain/{forecast,runs,jobs} endpoints, forecast_explanation table, and the ExplanationPanel UI. Dogfooded in a real browser.