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.

hscontrol/db/suite_test.go

@@ -34,7 +34,6 @@ func newSQLiteTestDB() (*HSDatabase, error) {
 				Mode: types.PolicyModeDB,
 			},
 		},
-		emptyCache(),
 	)
 	if err != nil {
 		return nil, err
@@ -95,7 +94,6 @@ func newHeadscaleDBFromPostgresURL(t *testing.T, pu *url.URL) *HSDatabase {
 				Mode: types.PolicyModeDB,
 			},
 		},
-		emptyCache(),
 	)
 	if err != nil {
 		t.Fatal(err)