Visualize: Demand Planner page + interactive Forecast/Backtest pages

[w7-mgfcode]

Implements PRP-22 (PRPs/PRP-22-visualize-demand-planner.md).

Extends the Visualize menu from two flat, read-only job viewers into a business-facing demand surface — the successor to PRP-20/PRP-21 (Explorer interactivity), applied to the analytics slice and frontend/src/pages/visualize/.

Scope (additive only — no migration, no new slice, no app/main.py change)

Backend

• GET /analytics/inventory-status — latest inventory_snapshot_daily row per (store, product) grain via Postgres DISTINCT ON; optional store_id / product_id filters; 200 + empty list on an empty table.

Frontend

• New page Visualize → Demand Planner (/visualize/demand) — multi-SKU table built from completed predict jobs: tomorrow / next week / next month demand + inventory requirement, with a lead-time selector and a single-SKU drill-in.
• Interactive Forecast / Backtest pages — in-page job submission, prediction-interval band toggle, CSV export, run/job cross-links.

Acceptance

See the PRP's Success Criteria + Final Validation Checklist.

Scopes: analytics / ui / docs.

[w7-mgfcode]

Resolved on dev by 4d7592f feat(ui): Visualize Demand Planner page + interactive Forecast/Backtest pages (PRP-22) (#196). The Demand Planner page, GET /analytics/inventory-status, and the interactive Forecast/Backtest pages are all merged. Closing as completed (PR #196 did not back-reference this tracking issue).