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.

cmd/headscale/cli/policy.go

@@ -28,7 +28,7 @@ func bypassDatabase() (*db.HSDatabase, error) {
 		return nil, fmt.Errorf("loading config: %w", err)
 	}
 
-	d, err := db.NewHeadscaleDatabase(cfg, nil)
+	d, err := db.NewHeadscaleDatabase(cfg)
 	if err != nil {
 		return nil, fmt.Errorf("opening database: %w", err)
 	}