Add product categorization feature

[devin-ai-integration]

Add product categorization feature

Summary

This PR adds comprehensive product categorization capabilities to the inventory management system, enabling better organization and filtering of products by category.

Backend changes:

• Created Category entity with JPA annotations (id, name, description)
• Added ManyToOne relationship from Product to Category (nullable foreign key)
• Implemented full CRUD REST API at /api/categories via CategoryController
• Added CategoryService and CategoryRepository following existing patterns
• Extended ProductController with /api/products/category/{categoryId} endpoint for filtering
• Updated DataInitializer to create 4 sample categories (Electronics, Computers, Audio, Wearables) and assign them to existing products

Frontend changes:

• Created new CategoryList component with interactive category cards and product filtering
• Updated Dashboard to display total category count as a new stat card
• Added "Categories" navigation link to sidebar with tags icon
• Extended API service with 6 new category-related functions
• Added routing for /categories path in App.js

Testing:

• Backend compilation: ✅ Passed
• Frontend build: ✅ Passed (1 pre-existing warning unrelated to this PR)
• Backend tests: ✅ All 5 tests passing

Review & Testing Checklist for Human

⚠️ Risk Level: YELLOW - Database schema changes with limited end-to-end testing

• Test full category workflow in browser: Navigate to http://localhost:3000/categories, verify categories display correctly, click on each category to filter products, verify products show up correctly for each category
• Verify database behavior: Check what happens when you try to delete a category that has products assigned to it (should fail with foreign key constraint or handle gracefully)
• Test Dashboard integration: Verify the new "Product Categories" stat card displays the correct count (should show 4 after initialization)
• Check existing product handling: Verify that products without an assigned category still display properly in the product list (category is nullable)

Notes

• Database schema change: This PR adds a new categories table and a category_id foreign key column to the products table. The relationship is nullable, so existing products without categories are supported.
• No explicit migration scripts: JPA/Hibernate auto-creates the schema in dev (H2), but production deployments may need explicit migration scripts (Flyway/Liquibase).
• Cascade behavior: Category deletion does not cascade to products - attempting to delete a category with products will likely fail with a foreign key constraint violation. This may need explicit handling in a future PR.
• Sample data: DataInitializer creates 4 categories and assigns them to the 5 sample products on first run.

Session: https://app.devin.ai/sessions/04a702452ace48b1a011a37aa9da125e
Requested by: mason.batchelor@cognition.ai (@mbatchelor81)

[devin-ai-integration]

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