REST API request: add a venue parameter to POST /calendar/events and POST /calendar/events/{id}

At the moment there's no way to link a calendar event to a saved venue through the REST API. Marc has confirmed this in Feature Support.

The API already knows about venues. Event responses include a venue object, and venues can be listed, created and edited through /calendar/venues. Only setting the link from the event side is missing.

What I'd like added:

1. A venue parameter (int, venue ID) on POST /calendar/events and POST /calendar/events/{id}, working the same as choosing a venue in the event form. Sending 0 would remove the link.

2. Ideally, a venue filter on GET /calendar/events, so integrations can list events by venue.

Why it matters:

- Venue pages: each venue's "Upcoming Events" block stays empty for API-created events, so venue pages can't show what's on. Likewise each event posted via the API is orphaned from it's venue.

- Structured data: events linked to a venue output location.name in their Event schema. API events can only send a location, so Google Search Console flags every one as "Missing field 'name' (in 'location')".

- Scale: we import hundreds of local events automatically each week, so linking them by hand in the edit form isn't realistic.

It looks like a small addition, since the venue relationship already exists in the data model and the front-end form. It would go nicely alongside the image upload support requested here.

Thanks!