Gig create/update rejects venue-linked gigs (Invalid create gig data)

[JoshuaVSherman]

Blocks WebJamApps/JaMmusic#1254 "cannot create new gig error needs fixing immediately!" — Josh cannot post a newly booked gig. Production defect, both repos already on main.

Why

Since JaM's 3-path venue picker shipped, the "Pick Existing Venue" path sends venueId and deliberately sends empty strings for the legacy free-text fields (JaMmusic/src/containers/Music/Gigs/CreateGigDialog.tsx:65-76 passes '' for city and usState as literals; venue is only ever populated by the "No Venue (one-off)" editor).

This server still hard-requires those legacy fields, so every venue-linked gig is rejected:

• src/AgController/index.ts:272 — if (gig && gig.datetime && gig.city && gig.usState && gig.venue) → else Invalid create gig data. This is the error Josh sees.
• src/AgController/index.ts:308 — updateGig has the identical check (Invalid gig data), and JaMmusic/src/containers/Music/Gigs/EditGigDialog.tsx:101-105 hardcodes city:''/usState:'' on every save, so editing a venue-linked gig is broken the same way — create just fails first.
• src/model/gig/gig-schema.ts:16 — venue: { type: String, required: true }. Even with the two checks relaxed, Mongoose rejects a venue-linked gig that has no free-text venue.

city/usState/venue are vestigial for the linked path: every consumer already resolves display values through the populated venue (JaMmusic/src/containers/Music/Gigs/gigs.utils.tsx:226-231, Gigs/index.tsx:26-55).

Scope

1. newGig (src/AgController/index.ts:272) — accept a gig when it has datetime AND (venueId OR a non-empty venue). Drop city/usState from the requirement entirely; they are display-only legacy and are resolved from the linked venue. Keep the same socketError/Invalid create gig data message shape on genuine failures.
2. updateGig (src/AgController/index.ts:308) — same rule, same reasoning. Keep the #253 rethrow behavior exactly as it is.
3. src/model/gig/gig-schema.ts:16 — venue becomes required: false. A gig is now identified by venueId OR venue; neither alone is mandatory at the schema layer, and the handler above is the gate.
4. Unit tests in test/AgController/index.spec.ts covering, for both create and update: venue-linked payload (venueId set, venue/city/usState empty) → succeeds; one-off payload (venue set, no venueId) → succeeds; neither venueId nor venue → still rejected with the existing message; missing datetime → still rejected.

Non-goals

• No change to the legacy newTour/editTour message-name tolerance — it keeps working as-is.
• No JaM/frontend changes. The companion frontend fix (EditGigDialog wiping city/usState on legacy gigs) is EditGigDialog wipes city/usState on one-off (no-venue) gigs JaMmusic#1275 "EditGigDialog wipes city/usState on one-off (no-venue) gigs" and is independent of this issue — neither blocks the other.
• No data migration and no backfill of venueId onto old gigs.
• web-jam-back/src/model/gig/gig-schema.ts:22 carries the same venue: required: true, but that process never creates gigs (gig-router.ts exposes only /:id/announce plus byId, and findByIdAndUpdate does not run validators by default), so it is dormant. Leave it alone here; a separate alignment issue can pick it up if it ever bites.

Acceptance criteria

• Creating a gig with only datetime + venueId succeeds and publishes gigCreated.
• Editing that gig with city/usState empty succeeds and publishes gigUpdated.
• A one-off gig (free-text venue, no venueId) still creates and updates.
• A payload with neither venueId nor venue still transmits socketError.
• Full npm test green (lint + typecheck + coverage), not sub-scripts. Coverage gate stays at or above its current level.