Bind this project to a Slack channel, or clear the binding with an empty channelId.

The binding is per project and there is no org-wide default to fall back on: an inherited channel is how one customer's Slack Connect channel quietly starts receiving another project's uploads. Saving also posts a confirmation into the channel — a mis-typed id is only ever caught by a human seeing a project name they do not recognise.

Request Body: projectId (int): required channelId (str): Slack channel id, e.g. "C09ABCXYZ". Empty/absent clears the binding. channelName (str): optional display name; looked up from Slack when omitted.