types: add node.expiry config, deprecate oidc.expiry

Introduce a structured NodeConfig that replaces the flat
EphemeralNodeInactivityTimeout field with a nested Node section.

Add node.expiry config (default: no expiry) as the unified default key
expiry for all non-tagged nodes regardless of registration method.

Remove oidc.expiry entirely — node.expiry now applies to OIDC nodes
the same as all other registration methods. Using oidc.expiry in the
config is a hard error. determineNodeExpiry() returns nil (no expiry)
unless use_expiry_from_token is enabled, letting state.go apply the
node.expiry default uniformly.

The old ephemeral_node_inactivity_timeout key is preserved for
backwards compatibility.

Updates #1711

hscontrol/app.go

@@ -583,7 +583,7 @@ func (h *Headscale) Serve() error {
 
 	ephmNodes := h.state.ListEphemeralNodes()
 	for _, node := range ephmNodes.All() {
-		h.ephemeralGC.Schedule(node.ID(), h.cfg.EphemeralNodeInactivityTimeout)
+		h.ephemeralGC.Schedule(node.ID(), h.cfg.Node.Ephemeral.InactivityTimeout)
 	}
 
 	if h.cfg.DNSConfig.ExtraRecordsPath != "" {