Add restocking feature with budget recommendations and order submission

[Ge0f3]

Summary

Adds a Restocking feature: users set a budget, get prioritized restock recommendations, and submit restock orders that persist to disk. Submitted orders then surface on the Orders page.

Backend

• GET /api/restocking/recommendations — budget required; supports warehouse/category filters
• GET|POST /api/restocking/orders — the only write endpoint; persists to server/data/restock_orders.json
  • Atomic write (temp file + os.replace) so a crash mid-write can't leave truncated JSON
  • A lock serializes the read-modify-write so concurrent POSTs can't collide on ids
  • Writes to disk before publishing in-memory, so a failed write can't leave a phantom order visible in the API

Frontend

• New Restocking view + route and api.js client methods
• Orders page gains a Submitted Orders card. It intentionally ignores the global filter bar (the year/status options don't apply to just-submitted, multi-warehouse restock orders) — matching the existing precedent of /api/demand and /api/backlog
• Its own error state so a not-yet-restarted backend degrades just that section instead of blanking the whole Orders page

Other

• en/ja i18n strings
• Architecture docs under docs/
• Backend test suite (tests/backend/test_restocking.py)

Testing

• Verified all 7 nav tabs load via Playwright; Restocking route renders
• Backend test suite added for the new endpoints

🤖 Generated with Claude Code