docs/gig-outreach: document entity relationship diagram (ERD) and data dictionary across gig, venue, outreach, and report models

[JoshuaVSherman]

Child of web-jam-tools#875 ("book-gig skill phase 2 enhancements (Epic)").

What this builds

Documents the comprehensive Entity Relationship Diagram (ERD) and data dictionary across all gig-outreach models (Venue, Gig, Outreach, Template, and OutreachReport) in both the master design document (~/Dropbox/web-jam-llms/gig-outreach/book-gig-skill-design-2026-08-16.md) and in-repo architecture documentation (skills/book-gig/README.md or docs/gig-outreach-erd.md).

1. Adds complete Mermaid ERD showing entities, attributes, primary/foreign keys, cardinality, and embedded subdocuments (touches, followUps, suggestion).
2. Documents foreign key resolution rules (Gig.venueId -> Venue._id, Outreach.venueId -> Venue._id, Outreach.templateUsed -> Template.type).
3. Documents spacing and calendar intelligence mechanics (Venue.gigInterval, Venue.resumeBooking, Venue.bookedThrough, Venue.bookingStatus).
4. Documents database-driven report persistence (OutreachReport keyed by weekend).

Acceptance criteria

1. Mermaid ERD diagram renders accurately in GitHub markdown.
2. Master design document (~/Dropbox/web-jam-llms/gig-outreach/book-gig-skill-design-2026-08-16.md) includes the ERD and updated filed issues table.
3. In-repo documentation provides data dictionary and field reference for developers and AI agents.