Backend handbook pages

Server routes, domain logic, runtime behaviour, and background work. 4 pages.

1. 01Packbase serverHow the Packbase API is structured, registered, authenticated, tested, and connected to data and background work.
2. 02Admin and moderation workWhere Packbase staff and moderation features now belong, what already exists, and how to preserve server-enforced authority.
3. 03Server and API changesA safe workflow for Packbase routes, schemas, authorisation, domain logic, tests, and public contracts.
4. 04Background workHow Packbase queues, imports, cleanups, retries, and graceful shutdown affect implementation and incident work.