Domain Model Migration

[onelrian]

• Priority: HIGH | Effort: LARGE | Dependencies: TICKET-002
• Acceptance Criteria:
  • Existing group.rs logic integrated into domain layer
  • Existing history.rs logic adapted for database
  • Constraint system abstraction implemented
  • Assignment engine with database persistence
  • User context integration in assignment operations
  • Backward compatibility maintained
  • Audit trail implementation
• Files: backend/src/domain/assignment/, backend/src/domain/history/