Add reserved seating to our events ticketing platform

Existing ticketing platform (Node/Postgres/GraphQL, live with 40 venues) currently does GA only. Add reserved seating: venue seat-map editor (SVG), seat selection in checkout with 10-min holds, and concurrency-safe allocation (two buyers, one seat — never). This is the hardest problem on our roadmap; I need someone who's done inventory/holds before. Code review with our lead weekly.