feat(simnet): add SimNet protocol end-to-end support

- model: SimNet protocol fields + NormalizeSimnet; type+port protocol uniqueness
- server config: OmnXT runtime config delivery via compatible() simnet case
- credentials: derive per-user psk/key_id from subscription (pkg/simnet), no new table
- subscription: adapter buildOmnxtSimnetConfigs + base64 buildOmnxtProtocolLinks + OmnXT SimNet application (migration 02161)
- UA gating: hide experimental protocols from non first-party clients (download + JSON node-list)
- admin: normalize simnet on create/update and on GET responses
- tests: 21 simnet unit tests; full suite green

internal/model/node/model_test.go

@@ -63,6 +63,7 @@ func TestAllProtocolsContainsKnownProtocols(t *testing.T) {
 		"tuic",
 		"hysteria",
 		"hysteria2",
+		"simnet",
 	}
 	set := make(map[string]struct{}, len(AllProtocols))
 	for _, p := range AllProtocols {