docs(api): update OpenAPI spec for v1 API
Update the OpenAPI specification to version 2.0.0 to document the
new v1 API endpoints:
- Add /api/v1/links endpoints (GET list, GET by ID, POST, DELETE)
- Add /api/v1/quotes endpoints (GET list, GET by ID, POST, DELETE)
- Add /api/v1/stats and /api/v1/users/{user}/stats endpoints
- Add /api/v1/search endpoint with type filtering
- Add /api/v1/cache endpoint for cache management
- Add /api/v1/kittens/daily endpoints (GET, PUT, DELETE)
- Add /r/{id} redirect shortlink endpoint
Other changes:
- Add X-API-Key security scheme
- Add reusable schema components (APIError, APIMeta, etc.)
- Use snake_case for all response field names
- Add tags for endpoint grouping
- Mark all legacy endpoints as deprecated