state: replace zcache with bounded LRU for auth cache

Replace zcache with golang-lru/v2/expirable for both the state auth
cache and the OIDC state cache. Add tuning.register_cache_max_entries
(default 1024) to cap the number of pending registration entries.

Introduce types.RegistrationData to replace caching a full *Node;
only the fields the registration callback path reads are retained.
Remove the dead HSDatabase.regCache field. Drop zgo.at/zcache/v2
from go.mod.