feat(ui): Visualize Demand Planner page + interactive Forecast/Backtest pages (PRP-22)

[w7-mgfcode]

Implements PRP-22 (PRPs/PRP-22-visualize-demand-planner.md). Closes #195.

Turns the Visualize menu from two read-only job viewers into a business-facing demand surface — the successor to PRP-20/PRP-21 (Explorer interactivity).

What changed

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

• 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. New InventoryStatusItem/InventoryStatusResponse schemas + compute_inventory_status service method.

Frontend

• New page Visualize → Demand Planner (/visualize/demand) — every completed predict job rolled into a multi-SKU table (tomorrow / next-week / next-month demand + inventory requirement), a 7/14/30-day lead-time selector, client-side sort, CSV export, and a single-SKU drill-in (demand curve + reorder breakdown + cross-links).
• TimeSeriesChart gains an opt-in prediction-interval band (LineChart → ComposedChart, backward-compatible — new props default off).
• Forecast / Backtest pages made interactive: in-page job submission, interval-band toggle (forecast), CSV export, run/job cross-links.
• New pure module demand-utils.ts (rollups + inventory math) with full vitest coverage; new useInventoryStatus hook; 3 new TS types.

Validation

• ruff / ruff format / mypy --strict / pyright --strict — clean
• pytest -m "not integration" — 1105 passed; analytics integration tests — 9 passed
• pnpm tsc --noEmit / pnpm lint / pnpm test --run — clean (44 tests)
• Dogfooded in a real browser (headless Chromium): Demand Planner table + lead-time recompute + drill-in, in-page forecast run, both upgraded pages — 21/22 checks, 0 console errors. Screenshots captured.

Note: dogfooding caught a real bug — /dimensions/{stores,products} cap page_size at 100, but the PRP blueprint specified 500/200; fixed to 100 before commit.

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