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.

flake.nix

@@ -27,7 +27,7 @@
         let
           pkgs = nixpkgs.legacyPackages.${prev.stdenv.hostPlatform.system};
           buildGo = pkgs.buildGo126Module;
-          vendorHash = "sha256-G+yhItFhlp2XP/Zd9N4nMQf96YMQLuYd069H+Quewtk=";
+          vendorHash = "sha256-x0xXxa7sjyDwWLq8fO0Z/pbPefctzctK3TAdBea7FtY=";
         in
         {
           headscale = buildGo {