Refactor bracket validation logic to support draft state

This commit updates the bracket validation process by introducing a new `ValidateDraft` method, allowing for incomplete edits in the draft state. The existing `Validate` method has been modified to enforce complete validation only upon confirmation. Additionally, new tests have been added to ensure that incomplete drafts are accepted until confirmation, while still rejecting invalid references. Documentation has been updated to reflect these changes in the API and architecture.

backend/internal/application/workflow.go

@@ -485,7 +485,7 @@ func (s *Service) UpdateBracket(ctx context.Context, actor domain.Account, event
 		return draft, err
 	}
 	draft.Matches, draft.Confirmed, draft.Version = matches, false, draft.Version+1
-	if err = draft.Validate(); err != nil {
+	if err = draft.ValidateDraft(); err != nil {
 		return draft, err
 	}
 	out, err := s.Store.SaveBracketDraft(ctx, draft, expectedVersion)