Populate linked venue on the gig feed (gig↔venue linkage)

[JoshuaVSherman]

What

Wire venue linkage into the SocketCluster gig feed so each gig carries its linked venue's live details.

• Add a venueId (ObjectId, ref: 'Venue') field to the gig schema.
• Add a minimal Venue model so the ref resolves.
• Make gig find / findSort .populate() the linked venue's fields.

Why

Part of the gig↔venue linkage effort. The JaMmusic side consumes this:

• JaM#1220 (shipped) regenerates the venue link at render time so venue renames/website/address fixes propagate to every gig — that needs the feed to deliver live venue fields.
• JaM#1242 (Gigs Location column) needs the venue's postal address delivered with each gig.

Status / provenance

WIP for this was found uncommitted on dev since 2026-07-17 (orphaned — no branch/issue/PR, likely a backend session cut off before commit). It's been rescued onto branch wip/gig-venue-feed-populate and is coming in as a draft WIP PR against this issue.

Remaining before merge

• Add a street address field to the Venue schema + include it in the populate (current WIP only carries name/city/usState/website — JaM#1242 needs address).
• Confirm JaM#1220's render-time propagation actually consumes this feed shape in prod (it may have shipped depending on this piece that never landed).
• Review + tests + typecheck (WJSC chains typecheck into npm test).