Reapply "Build post chat and attachment workflows"

This reverts commit 5bc4a551af.

backend/internal/users/invitations.go

@@ -25,6 +25,8 @@ type Invitation struct {
 	CreatedBy     *string    `json:"created_by"`
 	CreatedAt     time.Time  `json:"created_at"`
 	InvitationURL string     `json:"invitation_url,omitempty"`
+	EmailSent     bool       `json:"email_sent"`
+	EmailError    string     `json:"email_error,omitempty"`
 }
 
 type CreateInvitationInput struct {