Feature/wishlists notify me restock & price drop alerts

[heet2312]

Description

Implements the Wishlist + "Notify Me" (Restock & Price-Drop Alerts) feature requested in Issue #21.

Shoppers can now save products to a personal wishlist and subscribe to email alerts when an out-of-stock item is restocked or when a product's price drops below a chosen threshold or percentage. Alert delivery uses a console transport in development and SendGrid in production, and is gated behind FEATURE_ALERTS=true.

Fixes #21

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Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update

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How Has This Been Tested?

Unit tests — 29 new Jest tests in backend/__tests__/wishlist-alerts.spec.js covering:

• Wishlist GET / POST / DELETE endpoint validation and happy paths
• Alert subscribe validation (missing type, invalid type, price_drop without threshold, duplicate active sub)
• Alert cancel and list (mine) endpoints
• evaluateSubscriptions service: restock fires/no-fire, price_drop targetPrice fires/no-fire, dropPercent threshold fires/no-fire, idempotency (concurrent trigger is a no-op)

Manual testing:

1. Sign in → navigate to an out-of-stock product → click "Notify me when back in stock" → subscription created
2. Run PATCH /api/products/:id with { "stock": 10 } → backend console logs restock email ✅
3. Sign in → navigate to an in-stock product → click "Price-drop alert" → set target $900 (current $999)
4. Run PATCH /api/products/:id with { "price": 850 } → backend console logs price-drop email ✅
5. Repeat step 4 → no second email (subscription is TRIGGERED, idempotent) ✅
6. Wishlist add/remove → heart icon fills instantly (optimistic update), count badge updates in header ✅

To reproduce:

# Start both servers
cd backend && npm start
npm start

# Update stock to trigger restock alert
curl -X PATCH http://localhost:5000/api/products/<id> \
  -H "Content-Type: application/json" -d '{"stock": 10}'

# Update price to trigger price-drop alert
curl -X PATCH http://localhost:5000/api/products/<id> \
  -H "Content-Type: application/json" -d '{"price": 850}'
` ``

- [x] Unit tests
- [ ] Integration tests
- [x] Manual testing

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## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my feature works
- [x] New and existing unit tests pass locally with my changes

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive Wishlist and 'Notify Me' alert system, allowing users to save products and receive email notifications for restocks or price drops. Key additions include new Mongoose models, API endpoints for wishlist and alert management, and a SendGrid-powered email service. The frontend is updated with a dedicated wishlist page, a React context for state management, and interactive UI elements on product detail pages. Review feedback highlights opportunities to optimize the email service initialization, improve input validation for product updates, reduce unnecessary network traffic by removing wishlist polling, and ensure data consistency by fetching user emails dynamically during alert evaluation.

[heet2312]

All 5 Gemini review suggestions addressed in commits 9ce8f1d and 524be11.