Add models.go with all data structures:
- Checklist struct with fields for ID, Name, Public, Archived, etc.
- Task struct with full task data including status, priority, due dates
- TaskStatus enum (StatusOpen, StatusClosed, StatusInvalidated)
- Note struct for task comments
- User struct for user information
- Tags type as map[string]bool for efficient lookup
- DueDate struct with smart syntax support and constructors
(DueAt, DueString, DueInDays) and constants (DueToday, DueTomorrow,
DueNextWeek, DueNextMonth)
Closes checkvist-api-e9p
