chore(venue): one-time import — Gig Booking Worksheet xlsx → Mongo

[JoshuaVSherman]

Part of #818. Depends on the Venue collection + CRUD API issue.

One-time migration: import the Dropbox master worksheet into the new Mongo venue collection.

• Source: ~/Dropbox/joshandmariamusic/Gig Booking Worksheet 2025.xlsx (Dropbox master, ~79 rows, multi-section human-curated sheet).
• Map worksheet columns → venue fields; preserve status, notes, contact info.
• Idempotent — rerunnable via the create upsert (no duplicate rows on re-run).
• Run, verify row counts + spot-check a few venues.

This unblocks the cutover issue (deleting the xlsx files). Do not delete any xlsx until this import is verified.