Set up new crates for production system

Since the plan is to keep the PoC running for a while next to the production version of buildbtw, create some new crates next to the existing ones:

buildbtw
# old: 
├── arch-pkg-repo-updater
├── buildbtw-poc
# new: 
├── repo-manager # Pacman repository storage
├── backend # API, scheduling, integrates repo-manager
├── api # definitions for client to backend communication
├── worker # for local builds
├── gitlab-executor # gitlab runner custom executor
└── client # client

These should get the necessary tasks in the Justfile as well. We should rename the PoC justfile tasks to include "poc" in their names, e.g. just run-server -> just run-poc-server.

For consistency, let's name the new client bbtw and rename the old one to buildbtw-poc-client.