Document all API endpoints

[RossCabrera]

Overview

Add comprehensive docstrings and examples to all API endpoints for Swagger
documentation and developer reference.

Documentation Requirements

For Each ViewSet

• Class docstring explaining purpose
• Method docstrings for each action
• Parameter descriptions
• Request body example
• Response examples (200, 400, 401, 403, 404, 500)
• Error codes and messages

Endpoints to Document

• Authentication (registration, login, Google OAuth, password reset, etc.)
• Notes CRUD
• Categories CRUD
• Token refresh

Acceptance Criteria

• All endpoints have docstrings
• Swagger shows descriptions
• Examples provided
• Error codes documented
• Authentication method documented
• Parameter types clear
• Response schemas clear
• Readable in Swagger UI

Related Issues

• Depends on: Set up drf-spectacular for Swagger docs #26
• Blocks: Add README with setup instructions #29