Expired backup archives are deleted again

v0.45.1-beta

Expired backup archives are deleted again. The agent did receive the purge order the platform sent, but did not recognise it and ignored it silently: old archives therefore stayed on disk indefinitely, and the platform waited a minute for an answer that never came, on every retention cycle and for every project. The command is now routed correctly, and a test automatically checks that every command the agent knows is routed all the way to its handler, so this kind of omission can no longer go unnoticed.