Investigate: expand gig announce to more promotional endpoints (local event calendars from venue-mining sources, band-gig platforms)

[JoshuaVSherman]

Context

Follow-on to #962 / JaMmusic#1209 (one-click gig announce → Instagram + Web Jam LLC Facebook). Josh wants to know where else a booked gig could be promoted with acceptably little friction — ideally more legs behind the same Announce button, or a checklist the dialog surfaces.

Investigate (deliverable = findings comment on this issue, then close)

1. Local events publications — the venue-mining registry (sources.yaml next to the /venue-mining skill in web-jam-tools) already catalogs per-metro local events publications from the venue-discovery work. Many such sites accept public event submissions. For the metros where Josh actually gigs: which of the cataloged sources take event submissions, via what mechanism (form / email / API), and which could be automated vs. belong on a manual checklist?
2. Band/gig platforms — evaluate the current (verify, don't trust memory) submission/API story for artist-gig listing services: Bandsintown for Artists, Songkick, and similar. Do they have APIs or bulk tools an announce leg could call?
3. Other candidates — Google Business Profile posts, Nextdoor, community FB groups (likely manual-only — confirm), anything else that surfaces during research.

Ground rules for the eventual design

• Same philosophy as Gig announce endpoint: POST /gig/:id/announce → Instagram + Web Jam LLC Facebook page #962: human clicks, preview-confirm, NO auto-send/cron, no scraping Facebook.
• Prefer few high-value legs over many flaky ones; anything without a stable API/mechanism goes on a manual checklist in the dialog rather than an integration.
• Findings should rank candidates by (reach for Josh's metros) × (automation feasibility) and recommend which, if any, become build issues.

Closeable when

Findings + recommendation are posted here and any approved build issues are spun out.