Don't empty the arena

hackademy-worker needs a kernel patch, so everything on it has to move. kubectl drain cordons the node and evicts its pods — and without a limit it evicts them all at once, which here would take two of the three servers down together.

A PodDisruptionBudget sets that limit. With minAvailable: 2, the eviction API refuses to evict an arena server while fewer than three are up, and drain keeps retrying until the replacement is ready.