Feature request: accommodations for event hooks

I'm trying to add some automation & verification to my workflow. Some examples:

• When a post is set to 'published', automatically set the 'published_at' field to the current time
• A post may only be published if required fields are filled in
• When a post is created, store a random nonce so I can pass around unpublished drafts with secret URLs
• Disallow saving a post if it's already published and the slug would change
• Alternatively, create a redirect if I change a post slug

I don't know what Keystatic plans for the long term, but I think right now I could do all of this with just a few small changes:

1. Commit to the /update route being a stable API so I can build a middleware that intercepts it
2. Include the collection name and slug as fields in the POST body to /update
3. Allow the server to signal that the client needs to reload the post content. Maybe a header?
4. Have the UI gracefully handle if the server returns an HTTP error status & error message when saving.

With these changes, my server can intercept post updates and change the content however it needs. Looking at the existing network traffic, this feels fairly non-intrusive, but offers big benefits.

Start by tracing the /update route, its POST body, and the UI's save handling using the existing network traffic described in the issue. Determine how collection and slug data, server reload signals, and HTTP error messages would fit together; done means the route contract and client behavior support these event-hook workflows.