Adds MultiCollectionController + RerunCollectionController (list/get/create/update/ delete) over the existing MediatR handlers, with response/request DTOs, controller + contract tests, regenerated OpenAPI + TS types, and thin SPA api-client modules. SPA editor screens are a follow-up. Rerun request DTO resolves the chosen entity id into the minimal VM the handler reads (documented inline). Refs #151, #152 Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>