Build Category ViewSet with endpoints

[RossCabrera]

Overview

Create Django REST Framework ViewSet for Categories with create, read,
update, delete operations. Implement user isolation and color validation.

Requirements

• Create CategorySerializer with color validation
• Create CategoryViewSet with CRUD operations
• Validate hex color format (#RRGGBB)
• Ensure name is unique per user
• Implement user isolation
• Handle deletion (notes.category becomes NULL)

Acceptance Criteria

• Users see only own categories (AC-NOTE-03)
• Categories have name and color (AC-NOTE-03)
• Color hex validation works
• Name unique per user
• Authenticated access required
• 404 for non-existent categories
• 403 for unauthorized access
• Deleting category sets notes.category to NULL
• Input validation (name required, length limits)
• Case-insensitive duplicate check

Related User Stories

• US-NOTE-06: Color-coded categories

Related Acceptance Criteria

• AC-NOTE-03: Custom category names and colors

Related Issues

• Depends on: Run initial migrations and test database connection #5 , Set up JWT with djangorestframework-simplejwt #8
• Blocks: Build workspace sidebar with categories #22, Build note grid/card view #23, Set up drf-spectacular for Swagger docs #26