Add Restocking recommendation engine with budget optimization

[adriengrimal-hue]

Summary

• Introduce new Restocking feature with AI-driven recommendations based on demand forecasts and available budget
• Smart budget optimization algorithm (greedy selection sorted by trend priority: increasing → stable → decreasing)
• New API endpoints for restocking recommendations, order submission, and retrieval
• Display submitted restocking orders in Orders page with expandable item details
• Full internationalization support (EN/JA) for new feature

Test plan

• Navigate to /restocking tab and verify page loads
• Test budget slider from $0 to $65,000 and verify recommendations update in real-time
• Submit a restocking order and confirm success message appears
• Verify submitted orders appear in Orders page "Submitted Orders" section
• Check expandable item details show correct quantity and unit costs
• Test language switching (EN/JA)

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