API handbook pages

HTTP contracts, authentication boundaries, SDK coordination, and compatibility. 4 pages.

1. 01Packbase overviewThe main Packbase concepts, request paths, repositories, and ownership boundaries.
2. 02Packbase serverHow the Packbase API is structured, registered, authenticated, tested, and connected to data and background work.
3. 03Server and API changesA safe workflow for Packbase routes, schemas, authorisation, domain logic, tests, and public contracts.
4. 04Cross-repository changesHow to coordinate Packbase server, SDK, web, and documentation changes without leaving an incompatible gap.