Add comprehensive authentication and authorization enhancements:
- tokens.go: New token management system for public task access and cloning
* SHA-256 hashed token storage for security
* Token generation, validation, and automatic cleanup
* Support for public access and clone permissions
- api_key.go: Extend User struct with Groups field
* Lab group membership (ml-lab, nlp-group)
* Integration with permission system for group-based access
- flags.go: Security hardening - migrate to structured logging
* Replace log.Printf with log/slog to prevent log injection attacks
* Consistent structured output for all auth warnings
* Safe handling of file paths and errors in logs
- permissions.go: Add task sharing permission constants
* PermissionTasksReadOwn: Access own tasks
* PermissionTasksReadLab: Access lab group tasks
* PermissionTasksReadAll: Admin/institution-wide access
* PermissionTasksShare: Grant access to other users
* PermissionTasksClone: Create copies of shared tasks
* CanAccessTask() method with visibility checks
- database.go: Improve error handling
* Add structured error logging on row close failures
