Update api/models/account.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

api/models/account.py

@@ -272,7 +272,7 @@ class InvitationCode(Base):
     status: Mapped[str] = mapped_column(db.String(16), server_default=db.text("'unused'::character varying"))
     used_at: Mapped[datetime] = mapped_column(db.DateTime)
     used_by_tenant_id: Mapped[Optional[str]] = mapped_column(StringUUID)
-    used_by_account_id: Mapped[str] = mapped_column(StringUUID)
+    used_by_account_id: Mapped[Optional[str]] = mapped_column(StringUUID)
     deprecated_at: Mapped[datetime] = mapped_column(db.DateTime)
     created_at: Mapped[datetime] = mapped_column(db.DateTime, server_default=db.text("CURRENT_TIMESTAMP(0)"))